Very timely... dd1 had a pony party for her 7th birthday on Sunday!
My dds ride every week anyway, but the friends who came were all complete beginners.
If your ds doesn't ride anyway, check out the stables. It should be registered with the BHS or ABRS (or both) and you should have an instructor who knows what s/he is doing with you (don't rely on the mums). Children who haven't ridden before shouls have someone walking beside them/leadingthem at all times. Children will need to wear comfortable trousers (leggings/tracky bottoms are better than jeans - the seams in jeans are too thick and tend to rub!), shoes with a heel (not wellies/trainers) and gloves. The stables should lend you hats (and sometimes boots too). When you ring up to inquire, ask whether all their hats conform to the most recent British safety standard.
All this sounds a bit 'formal' and stuffy, I know. But I wouldn't trust a place that didn't insist on all this... It would suggest a bit of a tin-pot operation to me, and I'd be a bit worried.
At dd1's party they did a bit of basic 'how to ride' stuff (walk, trot, stopping, steering) and then they played some games (races, etc.). It was great fun. The only slight downer was that it poured with rain the whole time. A stables with an indoor school would be ideal (ours doesn't have one!), but otherwise just be prepared to get wet, and bring a change of clothes if necessary.
HTH
(Oh, it was £20/child for an hour's riding.)