My YF has been doing the same thing, and I haven't had too much trouble with it. I also assume his parents may be reading, so I write the same way I would act around them, and I find that that works just fine. If he started to get particularly sappy I might remind him that what's said by IM isn't necessarily private.

Maybe your situation is different, but my YF's parents appreciate me being close friends with him. If he were to run up to me, hug me, and say "I love you", they'd think it odd if I didn't respond in kind. I do try to keep the tone as "between friends", though.