Turn the second bedroom into a dual purpose office/guest room. A trundle/day bed and your computer desk and files. The lounge can be combo lounge and music room.

When the Skid visits he stays in the guest room/office and when other guests visit they stay in tht room. Dump the bulky kid furniture and the rarely used toy collection.

End of problem.

The needs of the full time residents of the home trump the temp resident Skids.

IMHO of course.

It sounds like you agreed to fast to the smaller house without really sitting down and discussing how you were going to use the smaller amount of space. And whilst I can understand the want to have the piano there if you want to be giving music lessons, I am a little confused as to how such an item was overlooked if it really is THAT important to you.

My fiance and I have just moved into a smaller house; a 3 bed to a 2 bed, fortunately the living space is the same but even so, we have had to have various conversations about what to do with the bedrooms, at the moment it is just us plus SD4 but we want to have an ours baby (roughly in the next 3-5 years) so then what? To me those conversations are vital before moving to smaller living spaces but thats just how my mind works.

Now in regards to your SS bedroom, on one hand I can completely understand your want for an office/guest room but at the same time wanting to change SS bedroom to a guest room with a fold out bed because SS is there so infrequently seems a bit harsh. Lets face it, your going to have guests just as infrequently. Why should guests have higher claim to the house space than your SS?

However, as a compromise you might be able to get your partner to consider a new bed for SS that might solve part of the issue. You can get those high top beds that have a desk underneath but also a single seat sofa that opens into a bed (single) which will make use of some extra space. However only consider that if you are ok loosing that "office" space during SS's visitation. Im not sure on the size of your bedroom but is there not space for a desk in there? (We are lucky both bedrooms here are huge so we've been able to create an "office corner" in our own bedroom).

Also look into changing your downstairs sofa, there are a few great corner sofas that open up into double beds. Be careful though as the cheap ones are not comfortable. when we moved we discovered our sofa opens up into a double bed which solved a potential guest issue there.
And if that fails then buy a double air bed, and what you do is simple. If you have guests coming to stay and you want to give them a bed then give them your bed and you and hubby camp in the living room on the air bed. If you dont want to give up your own bed then inform them theres only a single bed in SSs room but youve got a double air bed that can be set up in the living room. Im guessing anyone that would come to stay with you knows you live in a two bed house, I doubt they would expect to be put up in bedrooms you dont have. If they are people that are THAT concerned about their comfort levels theres always hotels.

Point is, yes having a guest room is nice but its one of the things you can only do when you have the space. With the current set up you dont have the space.
You can possibly turn SSs room into his room/office area but even if you did that you will probably not free up enough space downstairs to get in a piano. (and if you did youd probably loose floor space for air beds anyway)

How long is your contract on this place? Im assuming its still rental? If the space is that important you might have to start looking for something more suitable and hold off guest room/piano plans until then.

Can you get a pull out bed. That way the SS gets his bed in his room for when he is there. However when you have guests you can pull out the bed. Maybe compromise and get rid of one book shelf...and out your piano there.

Not sure if you rent this new 2 bedroom or actually purchased it. If rented, the answer is simply to keep looking for a larger unit. There may be more openings now than when you initially had to move.

I'll assume there are many more reason why you believe/feel the child is "one of the most spoiled, babied, indulged and doted on kids" you have ever met. A child having a place to sleep and some toys alone does not equate the description. Several good observations have been pointed out by others above 1)'where does the piano and my office space go' when first considering the offer. 2)'will we be having overnight guest and if so how do we accommodate them'. You were living with your parents prior to this current move so again, I assume there was no immediate rush to latch on to this unit so quickly. However, now that it's done, doesn't mean you're trapped forever in a place that doesn't suit your needs.

This topic comes up here often. Many look at it in different ways. Yes, the 'what about guest room' is raised. As mentioned prior, guest do not likely visit more often than the EOWE child. So if you insist on the 'guest' thing, it does have to be worked around the regular visitor aka the EOWE child. While you can always tell guest you simply don't have much room but there are nice hotels nearby, you can not tell your SO you have no room for his child. Or you can work with the place you do currently have. Sit down with your So with a floor plan, current furniture and desired additional pieces (the piano). Jointly toss ideas back and forth. Surely there are some current things that could go and other things that could be replaced with better suited pieces for the space.

How do you not resent the kid? That is a waste of resentment and inner energy, so you be a grown up realist about the situation and put it into perspective. The kid exist prior to this move. The kid will exist if you move again. It's not the kid's fault the place is too small and it's not the kid's fault you and SO have different priorities. The kid is the easy target for you here but he's not the problem.

Miss, I am feeling your pain. I have a 2-bed house which SO moved into. His kids stay EOW and a night in the week. My spare room - which was my Reiki room and guest room - now houses the 2 skids when they stay. My parents who live 2 hours away and are in their 70s now can't stay at mine (they have to go to a hotel) and I have lost my Reiki room. However, my SO made sacrifices as well when he moved in. Until we move to a bigger house we are a bit stuck. But the fact that your skid only stays over 2 nights a month does mean that it is unreasonable that it is a permanent bedroom for him and I would think that with some flexible furniture it could be used more practically for more than one purpose. As other posters have said, look for a cabin bed (which would be fun for a young boy!) that you could use the space underneath, or a sofa bed which would be made up for him in advance of his arrival so it looks as if it is a permanent bed.