As long as he's enjoying the vocaloid doujin, that's all that matters. I freely admit that I enjoy my fair share of fanservicey schlock.
I think about my aunt who's a retired geriatric psychiatrist (who still does some consulting at her hospital). Because her job was so mentally taxing and draining, she never wanted to read anything too cerebral. She just wanted to let her brain relax. Hence why her bookshelf has lots of trashy novels that you wouldn't expect a learned person to have.
I don't claim that my job is as mentally taxing as that, but working in special education is psychologically draining in its own right. It's like, if I'm already sore from weightlifting that day, the last thing I want to do is exercise that night. So that's why I sometimes enjoy the more schlocky and less cerebral comics, manga, novels, games, anime, etc.