I think my boy has WHS

I'm not exactly sure how old Percy is, but I've had him a little over a year, maybe 14 months. I got him from someone needing to rehome. She said he was around 2 at the time, but not entirely sure. Lately, I've noticed him using his wheel far less and being less active overall. He seems to have trouble walking or even standing. He has moments where he looks okay, but doesn't seem to be doing well overall. I'm contemplating putting him down, but I obviously don't want to do that. Any advice at all would mean a lot. I haven't taken him to a vet for a formal diagnosis. Truthfully, I don't have the money to spare for one to say what I'm 99% sure is the case. For owners who have gone through something similar, when did you know it was time? I don't want him to suffer, but it's hard to let go :(