I wouldn't recommend getting him neutered... this kind of behaviour is unlikely to be caused by hormones, and neutering could well make things worse...
I do agree training is the way forward, and if the trainer who has been recommended to you uses stooge dogs to help that sounds great. Make sure the trainer uses positive methods, as any harsh training could make things worse in the long run.
Just to reassure you, the fact that he displays guarding behaviour to other dogs (guarding his most precious resource - you) doesn't mean he will react the same way to your baby. Our Honey can be difficult around some dogs, but has been great with our new baby - there's an element of jealousy, but not displayed in an inappropriate manner at all!
To prepare your boy for the arrival of your baby, make sure he is not in too much of a set routine - babies disrupt routines, and he needs to be prepared for this. Honey gets her main walk every morning, but that can be leaving at 8am or 11am, which means it doesn't matter too much if Isobel and I have a very slow morning. Also make sure your dog is happy to spend time apart from you - it sounds like this might be a challenge so get baby gates etc up NOW and get him used to gradually spending time relaxing on his own.
As far as the guarding behaviour, you might want to look at the book "click to calm" which has some great clicker exercises to help reactions to other dogs...