Your Darcy seems to be at a huge disadvantage here, never having learned the street rules/heirachy of the outside cat world from the beginning.

My suggestion is to buy him a textile collar and some feliway spray, to be applied on the collar, before he ventures outside.

This Feliway stuff is magic as far as I can tell. It really works with our two anyway. The male can get extremely aggresive towards our female, but once I plug in the feliway diffuser, all is well again within a very short time.
So maybe, by chilling Darcy out with the spray and by way of close proximity, any other cat getting close to him, perhaps he won't have such a hazardous time being out and about.
I haven't used the spray personally (hasn't seemed neccesary, they are both indoors), but going by how effective the diffuser works, I truly think it could be worth a try.
Good luck, maybe too, he just needs to learn some Swiss German or something, if he wants to intergrate well

Poor Darcy...