As a Light Gunner he can go to 3 regiments - 4 if you class Kings Troop (they can be light gun trained too). 7 Para RHA in Colchester, 29 Commando RA in Plymouth and Arbroath or 40 Regiment RA, currently in Topcliffe N.Yorks, soon to be based in Northern Ireland.
Presumably the carers office told him he was a light gunner?
He will go to 7 Para, 29 Commando, 40 Regiment (as above) or maybe 4th Regiment (keeping one lt gun bty post tour).
Or has elected to go light gun? Has he chosen light gun from the RA trades?
If he has, but isn't a cdo or para volunteer (which he would know - you have to volunteer to go to 7 or 29, you don't get sent there by accident) then that narrow it down to 4th or 40. If he has volunteered, then which one for?
If Para, then post Phase 2 he goes to Colchester and prepares for P-Coy. If Cdo he goes to Plymouth and prepares for All Arms commando Course.
Pirbright is nice. He should remember to invite you to his Families Day (about half way through the course - 7 weeks) and Pass Out parade (14 weeks after he started, unless he is back squadded).
If he can't tell you more, suggest he speaks to his RA Directing Staff at Pirbright. They should give more advice. All RA recruits also get an RA brief in the first 7 weeks, a visit to Larkhill and told which regiment they are going to before week 14.
If however, he just wont tell you more, because speaking to his Dad is like pulling teeth, then I can only hope you get enough info from here!
My last Adjutant was a Manx boy. He did imply that it was fairly boring on the isle, less for the mental bit surrounding the TT!
As your lad is at Pirbright, he should have received the briefs I mentioned - outline of the Royal Artillery and Regiments and maybe his Regimental choice (depending on how far he is through).
The Gunner presence here is strong and we are trying to keep the nurturing levels high, so if he is still confused or has random questions, assure him he can go and see any Gunner instructor for answers - they shouldn't bite!