2.5 is pretty young to be starting petite section, I think.
The very few kids who started at that age cried and clung when DS1 began PS, more than the bigger ones.
DS1 was 3.6 when he entered PS, as he is a March baby. He was bored at the nounou's by then, more than ready for school and even he clung a little and cried now and then over the first few weeks.
If your DS2 is just going for 2 mornings, that may paradoxically make it tougher, because he gets lots of days with you, and then there are only 2 mornings in the week when suddenly he needs to adjust to a whole other setup.
Starting in Jan may be harder, too, as lots of what is done in PS is to help kids learn the routine for life in a group; if most kids started in Sept, they'll "know" more about the rules and customs in the classroom than he does, and it may be discouraging for him. Perhaps it would easier to start at the start of the school year, when presumably the majority of kids in the class will be new, too.
If he seems really miserable and if you can keep him at home, maybe he'll be more ready in Sept.
(Disclaimer: in my town they really only take kids of 3 and over, due to lack of places in maternelle, and no one starts in Jan, so I have no experience of such a situation).