Depends how much money you have to throw at the problem - you can certainly get harnessed seats to 25kg e.g. http://www.securatot.co.uk/axkid-kidzone?category_id=61. I'd suggest talking to a car seat specialist, there's a lot more models around than are stocked at places like halfords & mothercare. Securatot are great if you can afford rear facing, or somewhere like Milton Keynes In Car Safety Centre or a local independent retailer. Many places will advise over the phone.
I would keep him in his old car seat until he exceeds the weight limit and/or look at seats that use a harness to a higher weight (there are some about but not sure of brands). He is a little young for just using the seat belt but my ds was the same and was out of his harnesses seat by the time he was 3 as he was over 18kgs.
My son's 2years & 4months he's broad child & is 17.5kg. The carseat he was in previously was only recommended up to 18kg so me and my husband went and got him a new one from halfords less than 2 wks ago which was fitted and we were told he would need a high back booster next, so this is what we bought, it was fitted & checked in our car like they do.
My son's been in the seat a few times and seemed fine until today when I had someone in the car with me and my son moved and managed to make the seatbelt slip off his shoulder and down his arm (he didn't get his arm completely out but appeared unsafe if I was to break) I'm now worried this carseat to old for him and his old ones unsafe due to his weight.
I was just wondering if anybody else had ever had this problem.