in Bari in Southern Italy with Polish troops at the end of WW11
It doesn't make much sense! Are you sure he was in the Polish army? If so, you can find his file. If not ....sorry.
Allies landed in Salerno and Reggio di Calabria - Bari is across the way of Southern Italy.
If he was there with any army it seems likely it was German Army. .but that wouldn't account for them turning him off, they would have taken him as a prison of war. unless all this is somehow mix up. Was he a deserter, a criminal escaping justice. I don't know, you may want to find out.
Why would he approach Americans? There was II Polish Corps. as a part of the Allied forces.
ended up escaping on a boat to UK via Algiers
Escaping what, whom?
The simplest way to find out the truth is to find what Army he was serving in - I'm sure there are relevant records in existence.