You said he's just over 15kg - how tall is he and how does he behave in his car seat? I think, even given poor muscle tone if he is tall, heavy enough and doesn't try and get out of his harness seat I think I would probably swap soonish to a 2/3 seat. What does his current seat go up to? I know the Britax stage 1 seats go up to 18kg.
Just another thought - my county council have a safety unit that goes around supermarket car parks giving advice and checking seats, you could give yours a ring and see if they have soemone who can give you advice.
Laura