Welcome to the board. There are, as you will discover, many, many of us in your situation so it's a good place to talk to others who 'get it' 
Worth talking to a few different providers, both organisations like UKYAP, PEACH, or consultants such as Sean Rhodes. Duncan Fennemore another 'big' name in Verbal Behaviour (strand of ABA). Each should tell you about what they do, how they work, what kind of approach might suit your ds and your family.
If you go for an ind consultant you would have to recruit the tutors yourself. We used Gumtree to start with. There is also an ABA tutor finder website (can't remember url but google it). And the aba yahoo group. The consultant would normally provide a supervisor for you. Generally, you would see the supervisor every few weeks, and the consultant every few months. This can be totally up to you, and how quickly progress is made, and of course how much you can afford! The cost does add up...
Depends on your circumstances of course, but we have done around 15 hours per week for a year and even with a relatively 'light' programme, have had amazing results (to the point where we may stop doing a full programme within the next few months).
Great news that you can start when ds is so young, it will really make a difference. Good luck.