I would just be on the lookout for any important life-altering decisions that he might make while still so immersed in NRE. If he suddenly has some wild scheme or crazy news, then I would tell him to slow down and get his feet on the ground. Otherwise, if he still spends time with you as frequently as he usually does, and hasn't let this relationship interfere with yours, I wouldn't worry too much.
As for not sounding jealous if you do wind up needing to talk to him, I guess I would start off any conversation by reinforcing how much you love and care about him, and have only his best interest at heart before explaining any concerns you might have. Also make sure there are no weird dramas or distractions in the immediate environment where you do have a discussion. You want to be able to communicate with his full attention on you, and in a calm serene manner.