i was married the first time to a man who felt very much the same way...
frequent, energetic sex was for only early on in a relationship.
he followed/s roy masters' philosophy. (that is a whole other thread
)
he too felt that there was a drain of "energy" with ejaculation (physically AND to a man's psyche energy).
it also had other issues about it because of past relationships and his fathers abuse (including sexually) to his mother....but i think the above mentioned doctrine was a good funnel for his lack of sexual involvement and to deal with it.
i tried to understand and be patient, but as a young woman, it was difficult physically and emotionally and very much contributed to some dysfunction in myself.
after a while i withdrew completely. when i tried to re-ignite any feelings i was met with disdain and rejection.
for that and a lot of other reasons the marriage collapsed.
unless he gets help/counseling to explore this further and the two of you can work out a compromise or a workable solution....you may be in for a world of hurt.
be grateful for the openess and honesty and keep that going....from both sides!
good luck.