Time to potty train?

So my son is 2 1/2 and is constantly taking his diaper off if he doesn't have pants on and this morning decided to take it off and dookie/pee in his crib (never done it before).. I have a toddler potty chair but he'll only stay on it for like 10 seconds then get off and doesn't like staying on it. Should I start potty training him? I never had this issue with my 7 year old and potty training him was pretty easy. Should I try a different potty chair? Picture of the potty chair I have. Any advice is helpful!