Ive tried a few.
Solgar, Solaray, Udo's Choice, Bioglan, Biokult, Biocare, Animal Parade, KIDZProbio
My favourite when they're little is the Solgar powder after a recall its hard to get
Solaray didn't like taste
Udos choice filled with crappy bulking agent
Bioglan - yummy chocolate ball. Fridge free. But has sugar.
Biokult- sachets a faff
Animal Parade- tasted chalky
Biocare- very annoying packaging (bottle) leading to spilt powder and it's the most exp powder
KIDZProbio- tasty
Natures Way- DS seemed happy
OHSO- for adults but mini choc bar with tiny squares making easy to split. Can get with sugar or without.
So I recommend for him KIDZProbio or Bioglan if he likes choc and you're not worried about a little sugar.
I've bought these from Holland and Barret, Amazon or Ocado