I definately agree with a behaviorist.Are you talking about rescue remedy.The drops given to calm down.I don't think that is a forever cure for agression.I have a friend that uses it to get new fosters used to a new environment if they are having trouble and at one point had to use it for her cat for a few days but not long term.I am not an expert on it though you would have to check with someone who is.I did once use a behaviorist on an aggressive dog and she was wonderful and very helpful..If you can't find one try to find a trainer trained in aggresion.Try, trainers,your vet,places like that to find a behavioist.It may just need to be an evaluation for one or two times that may be well worth a bit of a drive where the training could be done with someone else or you may, though I hope not, find you may need to give the him back.Good luck.
Laurie