Can I offer a slightly different take on this? To me it doesn?t sound as if your dog is aggressive, but that he is slightly dominant, and that he is wanting to protect the younger, and more vulnerable members of ?his? pack, i.e. your baby.
In dog society, the more dominant members of the pack will protect the young, and as this behaviour appears to have started since your baby was born this sounds like a plausible explanation to me.
I don?t actually think you need to rehome him immediately, but I do think there are steps you can take to try and iradicate this behaviour, and if these steps are not successful then rehoming is always an option.
Firstly, look at the way the dog is treated in the house. What areas of the house is he allowed? Is he allowed:
On the furniture?
Upstairs?
On the beds?
If the answer to any of these is ?yes? then you need firstly to stop him from going in any of these places. Allowing the dog to go on the furniture, upstairs, on any bed, puts him in the same hierarchical place in your pack as you, and he is therefore in a position to fight for dominance and will not submit to you as should be the case.
Secondly feeding. When and where are your dogs fed? Are they fed before or after you? If before, again this puts them in a more dominant place, because the more dominant members of the pack are the ones that get to eat first. This can be combated by either, making him wait until after you?ve eaten before he is fed, or if this isn?t possible due to timings etc, by you showing him you eat first, i.e. by eating a biscuit or something before you put the dogs? food down, so showing them that you eat first and are therefore higher in the pack than they are.
It?s possible he attacked the younger dog because he is more dominant than it, if he was there first then he had already established his place in his pack before it came along, and if it is not a dominant breed by nature then it might not have put up a huge fight for dominance.
Once you have re-established his, and your, places in the pack, you will be the more dominant one, and therefore the one that is charged with looking after the young, i.e. your baby.
As for the issue with the postee, if he?s teased the dogs before, then again he is seen as a threat, so taking away the threat, i.e. putting a postbox outside, will hopefully lessen the tention.
As he is such an old dog I really would try these things first before rehoming him. Give it a month, be very strict, and if there is no change then perhaps think again. But once you?ve started the changes these are things you need to keep in place to ensure he stays in his place in the pack.
Hth.