The ABA helpline would be a good place to start for information and support.
How long has it been since you last breastfed? How was the feeding going at that point?
Basically, if you had a full supply to start with and, for example, it's been one week since you stopped, then it will take about a week to get your supply back up.
is he happy to take the breast? If so, the best way to go about it is to feed him as often as possible. Are you able to take a few days out from other stuff to just spend time with him and feed? Perhaps you have family that can come and help out at home for a while.
If you get on to your local ABA group or a lactation consultant, they may be able to offer you some more ideas, and perhaps even discuss the use of a nursing supplementer system, which allows you to give your baby formula while he's at the breast - so he's getting milk and stimulating your supply at the same time. This can be particularly useful if your supply is very low and your son is reluctant to feed at the breast.
Another thing that helps some mums when relactating - and again, especially if baby is a bit reluctant to feed - is to express milk (which you can then use in the supplementer, as well) with a good-quality, hospital-grade pump (you can hire through the ABA).
Good luck!