It's a bit hard when he's under the weight requirements for the booster style seats.

I know exactly how you feel because we swap the seats over to DH's work car quite often. You just get super quick at it after a few goes.
For my 5yo we have a safe and sound pegasus which is better than the other boosters because it has the anti submarine clip as well at the shoulder strap guard. It has a 14kg weight minimum from memory though there is nothing on the seat instructions about the weight.