DS is a big lad and has grown out of his 'harness' type reclining car seat. He is now is a Group 2 full car seat which uses the car seat belt. Very quickly he learnt how to unbuckle the seat belt and I have had several occasions where he has leapt from the seat while I am driving and started running around my 7 seater car jumping on all the seats! He also unbuckles the seat belt when I stop the car and has great fun running from one end of the car to the other while I try to catch the little bugger (gritted teeth 
).
Patience is wearing thin now - how can I stop him unbuckling his seat belt PLEASE?