Put him back in a proper 5 point harness car seat.
I would not get one from Asda because they only have 18kg limit ones and these will likely be outgrown for you soon. What seat did you have him in before he moved to the booster? He should probably still fit into that, so no need to buy a new one.
If he is over 18kg (which is probably the weight limit for the old seat) or too tall for the old seat or you don't have it any more, Joie Bold has a harness up to 25kg. However it does cost about £180 which is a bit more than most of the Asda car seats.
If you can stretch to £100 there is Cozy n Safe Hudson as others have said but NOT the i-size one because this does not have the 25kg harness.
Or Babylo Enfasafe Event FX has a 25kg harness and is on amazon for £100 as well.
You need isofix+ top tether for all of those seats.
If £100 is too much, see if you can do his weight and height and I'll see if I can find you a seat that will fit him safely.
Def don't buy those dodgy harness things from random website - they don't offer any restraint and they can affect the position of the seatbelt