The new laws say that your DS has to be in a 5 point harness until he is 4. At 12 kg he is not heavy enough for a booster, they are only legal for over 14kg, and that is only if he is over 4 years old too. Your only real options at this point are to hire/ borrow a capsule and keep DS in his current seat (assuming it is a convertible one and not an actual infant capsule) or get a toddler seat like the maxi rider that goes from 8kg.