Allen,trap shooting can be very addictive for competitive folks,been class AA for 20 years now shoot occasional sporting clays with my poor man's trap gun,browning o/u sporting special I bought when my benelli and browning A-5 would'nt get me to the 25 straight I was gunning for,the purchase I made for this o/u I justified in my mind because it was the only o/u with a 3" chamber 30" barrel,figured it would be a good back-up shotgun or used at a game farm then the trap bug bit down hard had a special trap gun built/ordered from italy,sure enough moved up in the rankings after 10 to12k rds in practice every year then another 10k in ATA registerd events around the country.but as stated above browning BT-99 are a great starting trapgun,been around for many years,well proven work horse,I found shooting my field shotguns that after 20 rounds or less on hot day's the barrel would heat up distorting the target causing heat waves also this heat distorts the barrel diameter effecting the pattern ,the trap discipline that really got me hooked was shooting doubles,in oder to achieve good scores/average a semi is a gamble if your a registerd shooter,seen many old timer's on the doubles line drop more than one bird because of a malfunction not cycling each shooter is allowed one misfire after that its a loss bird,often puts a good shooter out of the money score wise but some folks are sensitive to felt recoil,then a semi is your huckle berry.
So if your son really gets in to the trap game and starts shooting all three events (singles,doubles and handicap) I would consider a good o/u with barrel select for starters but just shooting singles a (bt99 is a good starter will last for years),doing so the o/u will help with heat waves by shooting bottom barrel on singles,lowers felt recoil to shoulder,you can /should have the shotgun fitted by a professional for LOP,POI etc(length of pull,point of impact)
BTW are you reloading ammo or factory loads? most novice shooters tend to over amp trap load velocity,meaning they shoot high octane rounds like super sport or 1200fps rounds from the 16,this is a over kill for shooting singles and can result in the dreaded flinch.any cheap ammo such as rio,gun club,estate etc will break targets just fine from the 16yard line at 1140fps 2.75drams.This was a hard pill for me to swallow until I trained with one of the best trap shooters in the country.(Harlan campbell jr.)this guy is a trap shooting machine as the captain of the all american team at the time,pretty sure he's a senior shooter today still attending big shoots
also shooting trap our shotguns are not only fitted but have longer barrels 32" to 35" barrel length,this gives shooter a longer site plain and added weight to smoothin out the swing
just a few thoughts