Eva Ibbotson's YA books definitely! Funny, romantic, not explicit. Quite old-fashioned in a way. My favourites are "The Secret Countess" and "The Morning Gift".
Michelle Magorian (of "Goodnight Mr Tom" fame): "A Little Love Song" (does have 2 'sex' scenes but all very much implied and responsible, nothing explicit). She may have done others.
I Capture the Castle.
Anne of Green Gables - particularly the middle ones where she's in love with Gilbert, "Anne's House of Dreams", and "Rainbow Valley" and "Rilla of Ingleside" (second generation romance). Also her other series, Emily.
Louisa May Alcott - Little Women etc, but also "Rose in Bloom" and "Old-fashioned girl"
If he wants to work through a series then as a PP mentioned, Malcolm Saville's books in the Lone Pine series (and a couple of others) have developing romances between the friends. It's like Famous Five with teen romance. All very very subtle (they were written in the 50s). At that age I loved the fact you had to wait for and dig out the romantic subplots.
What Katy Did Next
And possibly, the EJ Oxenham's "Abbey Girls" series, almost all of which (school/adventure) stories have romantic subplots. Written from 19302-50s so all very low key.
Also second Jane Austen etc. I was reading her at that age though I struggled with Emma until I was older. I'd avoid the Brontes though, it's hard to understand the passion at that age.
I think it's great that he's reading these. Maybe he's a romantic, maybe he wants to be one of the leading men, maybe he's expressing something else. It might be worth seeing if he's interested in any books like "Will Grayson Will Grayson" or #ownvoices novels. That's a romance and comedy (and much more) from a teenage boy pov and features both straight and LGBT relationships.