...if he is an introvert (Google MBTI — half of the MBTI personality types are introverts, although not half the population), he may withdraw when he's feeling drained.
INFJ's — who are very rare — can actually love something so much that they have to get away from it to recharge (which is all part of the crazy, wonderful world of the terribly misunderstood but enormously influential INFJ type). If he's an introverted Thinker IxTx (much more likely), he may withdraw without having any idea that you would take it personally. He's would likely see it as just practical prioritizing — the work is now and you'll be there when he's done (right?).
When he "comes back up for air" you might want to try this: give him your blessing to withdraw as necessary, but just ask that he let you know he's planning to come back, and then let him do so. (This advice is actually from the trendy-to-hate "Mars and Venus" book by John Gray, but it's good advice when dealing with strong introverts of either gender.) My wife is an INTP and I try to follow
http://www.personalitytype.com/jyt/intp.html">this list of suggestions for making her happy:
* Respect my privacy and independence.
* Appreciate my competencies and wealth of creative ideas.
* Encourage me to spend time alone.
* Don't talk too much or force an emotional conversation before I'm ready.
* Try not to nag me about being messy or meeting deadlines.
* Above all - give me plenty of space to pursue my interests in depth and time to think things through.
I hope this helps. I could tell you a lot more if I knew his (and your) MBTI type.