Most pet stores claim their chins are four months or younger. I've talked to some wholesalers and their opinion is "if it LOOKS under four months I'll take it" so they could just be small chins that really are any age.
Him being caged with his sister won't help determine his age either. Petstores don't know or care about chins breeding at certain ages. They will throw whatever they want together and just turn a blind eye.
I still can't tell anything about his size from those pictures. At this point, you can just watch him, see how he grows and enjoy him. Don't worry too much about the age.