Is it possible to go to dog classes? This would help on two fronts. You and your dog will learn the commands and how to enforce them. AND your dog will get socialised properly with other dogs. Getting de-sexed may help the issue. Is there a reason why your DF doesn't want him done? A halti is a fabulous suggestion, my Lab wears one, she is nowhere near as well controlled with just a collar.

If you didn't have the pram with you I would suggest that everytime you are walking in a particular direction and he starts the behaviour, turn him around and walk in a different direction. When he stops the behaviour, you turn back the way you were going and continue, unless the behaviour starts again, in which case you turn around again - this can take several attempts to pas the one obstacle (dog / pedestrian / etc) but does work if you are consistant. Unfortunately that is very hard to do with a pram.

He really does need to be socialised more, because unfortunately, even if your dog is not intending to be agressive, you cann't guarentee how other dogs will react and you may be placed in a position where a dog fight does occurr and your dog gets the blame for starting it. (not fair, but it does happen).