My advice is simply to buy the boat which you feel is right for you and your partner. When looking for a boat you need to only consider your budget and the area in which you will be cruising. Will it be hot or cold, trade winds or not, deep or shallow water...that sort of thing. If you are sailing only around Boynton Beach and over to The Bahamas, a shallow draft monohull, preferably with a centre cockpit..or a cat would be the ant's pants.
Then consider equipment such as a water maker, solar array and wind generator. When it comes to the sailing boat proper, junior will easily adapt to anything larger than 10 feet, with one, two or three hulls.
What will be far more important is the sort of dinghy you buy. It will need to be stable, dry and reliable. It will need plenty of handholds, a smallish engine, oars and spray sheets. And, you will need to devise a safe, secure and dependable means of launching and retrieving the dinghy from your boat, and an equally safe and secure manner of transferring the young chap from one boat to the other, and back.
"if at first you don't succeed....Redefine success"!