I disliked those sort of books when I was a teen 
However I didn't mind the books where there was gentle coupling off, which was not much more than a close friendship with the implication of leading to marriage. It did tend to be older books 50s/60s. Would he go for those?
If you look for Girls gone by Publishing or on ebay for old versions, ones that I can think of offhand that have that sort of relationship are;
Monica Edwards: No Going back is probably the best, but the story is continued in (if I'm remembering rightly) Dolphin Summer, and lesser so A Wind is Blowing (hard to get hold of). There's also some romance in The Outsider, The Wild One (another hard to get hold of) and Fire in the Punchbowl.
Malcolm Saville: The Buckingham Series has Juliet chasing Charles certainly in the later ones. Diamond in the sky I think is the last and most obvious. Also the Marston Baines if he can cope with spy adventure on top of romance. Simon, in the style of James Bond, always has some girl on the go, although I think in the last (haven't read it GGBP hurry up and publish it!) they marry.
Lone Pine have the gentle romance alongside the mysteries: Not Scarlet But Gold, Man with Three Fingers, Where's my Girl and Home to Witchend have the most, again if I remember rightly.
Nancy Drew has her boyfriend Ned around in a lot of them.
I'm not sure those would really fit the bill. They're stories that some of the characters fall in love rather than romantic stories, but he might enjoy them.