I Have A Dotson That Is 13 Yrs Old .

I have a Dotson that is 13 yrs old . Yesterday he woke up ang could barely walk fell on his side afew times doesn't want to eat just drinks water, he seems weak not his normal self.I thought it might be his back but when I pick him up he doesn't hurt .He doesn't cry to go out we just take him out a few times a day I really need your advise on what to do for him