I think I should write to the Emla people and request they pay me as I advertise them on threads like this one 
Right, go to the chemists and buy Emla patch (or two if he's getting 2 jabs). I don't know if in the UK you need the prescription for that- don't think so. It's just a topical analgesic.
DS (8) had to have 2 boosters couple of months ago and was very scared- he didn't feel anything at all.
DD (23) mo has had it for most of her jabs and with mostly didn't feel anything and with a couple that she did (guess it depends on whether it's in the muscle vs under the skin)she cried for about 5 seconds as opposed to 5 minutes when she didn't have the patch.
So, I would tell him that he'll have the jab but also that he'll have a special patch that will make it not hurt.