I don't have a PB but I do have ACDs which is almost the same (in different ways) we are also often told not to take ours to the DP because of extreme prey drive & herding ability which is why I don't do DP's I also don't bc the amount of stupid ppl who frequent the ones in this area (you chazzers know what I'm talking about
) is enough that if I were to have kept going I prolly wouldn't have waist length hair :/.
@Barbara doesn't your DP have a seperate area for large & small dogs?
You mentioned he was most reactive when someone was holding the dog, what about starting off when a plush dog that resembles the kind he's reactive to so here (at first) is no danger of him hurting a dog? Then play the "look at that!" Game (I'm terrible at explanations but maybe someone else on here can or you can google it) kiko pup on you tube has a lot of gret stuff as does Sophia yin for reactivity.
Izze got for some reason reactive when her tail was touches when she was in her teens (went to the vet, no probs) so I started playing a tail game where when she wasn't stimulated I would touch it & then imediately ask for a sit & it wasn't long before she would sit when I touched her tail instead of snapping at me, even when stimulated, in fact it turned into her "calming button" lol.
I don't know if that'll work for you but anything is worn a try.