Here's the single most important thing for you to consider for your son for the next two years. He has absolutely no guarantee of anything from that coach and school to which he committed. The entire advantage of his commitment is with the school right now because your son is "off the market" with respect to other schools and is not forging any relationships with any other coaching staff or program.
Until he signs his NLI in two years, he really doesn't have anything other than a non-binding "promise" that if he continues to progress at the projected rate over the next two year, that he will sign a scholarship at what was committed to now.
Realize that the coach and staff MAY not even be at the school in two years and that any incoming staff will have no obligation to even look at your son, yet alone honor a verbal commitment made. Look at what occurred at Auburn and Alabama State recently on commitment "flips" when there was a staff change (both under very different circumstances).
Realize that the staff WILL continue to monitor your son's progress and may modify their offer as well as withdraw completely at their discretion. They will continue to make offers to other players that play the same position as your son's and, if they can "do better", they will.
You need to have your son continue to do everything he has done to this point to get the attention and offer he received.
Congrats and best of luck.