I had (have) a very similar situation with my neighbor. Even after showing him advance plans for my pool and him not objecting to anything at the variance hearings, he went ballistic after the pool was put in, even though we didn't encroach on his property at all and met all permitting requirements (and have a beautiful pool).

I finally got him to back off after threatening (obliquely) to have his above ground pool removed (it is against our deed restrictions) - he was being very vicious with the building inspector (who should have just ignored him, but didn't). Now, he just glares at me. But, other than exclaiming loudly (only sometimes) how wonderful the warm, solar heated, large, beautifully landscaped pool is, we pretty much ignore him, but are always courteous. Screw him, I can't spend my life being angry, I've got a wonderful pool to swim in!!!