Timmy can't groom his lower half

So a few weeks ago I noticed one of Gabby's boys was really slowing down. He has always been the active/athletic one of the 3 boys so it was quit obvious when his energy decreased..that didn't concern me to much considering they are almost 16 months old and pet store genes which we know usually aren't the best. But the past week or so I have notice when he tries to sit up to groom his back end he falls over..he just can't seem to balance like that..I have done the cheerio test and he does fine..he even still jumps on and off the couch when out..but he can't groom his bottom half and he hates it..I can see the frustration in his face when he falls over..any clue what can be causing this?

Is he a bigger rat? The bigger ones can't clean themselves as well. Next time you see him grooming, see if you can cup him or place him in such a way that he wouldn't fall.
Here's my mom's rat Sven, grooming himself using my dad. He had a back leg amputated so grooming wasn't easy for him.

He isn't a big rat he is actually just a little bigger than the girls haha he only weighs 499 grams..

Does he have full motion of everything? I normally don't go by rats weights weather they are chubby or not. I go by how they look cause some of my rats are sleek but heavy from muscle. I would watch him and see if you notice anything odd other then what you have already. If your really concerned I would take him to the vets just for a check up.

With 23 rats I can't just take them for checkups unless I am sure something is wrong..thus why I come here he is fine other than that..

It's hard to judge without being able to see what he does. Is he able to curl up in a ball when he sleeps? Does it look like anything gets in the way. It is possible it has something to do with his spine.

I have noticed balance problems such as you have described in my older rats that developed hed
and as previously suggested they needed some support re balancing when grooming

This is not a symptom of hed etc, but if you think something like that might be happening,
here is an interesting article http://ratguide.com/health/musculoskeletal/degenerative_osteoarthritis.php

Today I give Timmy a good wipe down because his backside was disgusting and his tail I don't even wanna talk about haha I thought he didn't groom it before but now seeing what it looks like not good I believe he was grooming it before he seemed to enjoy his wipe down and even participated and I held him up so he could help.. He seemed so grateful it made my heart smile.

Hopefully this works... It isn't him grooming he falls to fast so unless I'm ready and waiting and know he is gonna try to groom I miss it.. But this is the after affects of him trying to groom his lower half and fallen over.. He has trouble even getting up now if he lays a certain way like here on his back poor guy.https://youtu.be/xl8P4fhhHaE