Get him to have a cool bath then lie on his front with a fan on and maybe a damp sheet over his back. I don't think he'd need a + e unless he's losing an awful lot of fluid from the blisters or has heatstroke.
I would guess Ricror, only take him if he becomes unwell as a result of burning. If he becomes nauseous or has shivers, a trip might be worthwhile. If it his skin you are concerned about , a trip out for after sun cream. And when he's better give him what for , daft so and so .
he could go to the chemists and get some ointment for sun burn, the pharmacist can also advice on anything else he can use, also tell him to fill a bath with cool water, if he can put some ice in the water so much the better, yes i know this is from some experience years back. Unless he is being desperately sick, and it sometimes happens, i wouldn't think it necessary for him to go to A&E.
when we were in Antigua my daughter got a bad spot of sunburn on the top of one shoulder, the ladt in the massage place went out the back and picked some aloh vera and ground it in a pestal and mortar with ice and then applied it to the burn, it was amazing, instant healing almost