Hello! If you're going to go with goldfish in the future, definitely get a large tank and stick with fancy (double tailed) varieties like fan tails, orandas, telescope eyes, etc. You could have 2 in a 30 gallon tank, and they would be happy in there. If you wanted more, the general rule of thumb is 20 gallons for the first fish, and an additional 10 per additional fish, and make sure you double filter your tank. Now- down to the ASD stuff- does your son have any sensory sensitivities? Bright lights or colors, mainly. Also, quick movements? If you didn't want to deal with the heavy waste and high tank maintenance of goldfish in the future (they produce a lot of waste, and need very frequent water changes and etc), and he is fine with bright colors and fast movements, you could look into a tank of glofish- they are VERY active, tend to shoal a little bit, and are very bright, almost glowing colors.
http://www.google.com/search?q=glof...aLcaE2wXY1OyjBg&ved=0CHIQsAQ&biw=1280&bih=680 You'd need about a 20 gallon tank for them, minimum, and they are low maintenance.They're a community fish, as well, so you could put a couple of larger, slower moving fish in with them for variety. (like an electric blue dwarf gourami, maybe?) For your tank now, you could probably do a nicely colored male betta. They tend to be fairly interactive with people, after they get used you... will swim up to the glass and beg for food, etc. They are a little on the slower moving side, and will sometimes tend to just hand out in one spot- but, you can coax them into being active pretty easily by making them think they are about to be fed, and giving them a little treat. They are also super low maintenance, and very hardy... great beginner fish.