I got it on my 8th time having sex with him (as in my counter currently read 7 in the 'love life' tab).
@tjak1 - You have to have Hikaru wear the cat costume at least once, through I think you actually have to have him wear it a few times before the scene triggers. I had him wear it every time we had sex after the firework festival, after the first time he made no comment on it, till the eighth time where it triggered a dialogue and the scene where they both wear cat costumes. As for the chibi scenes, a lot of it is either combinations or randomness. Like, if you want to get the blow job scene, use the mouth icon on his dick three times in a row (If he's a dominant top, getting the blowjob chibi will cause the 69 chibi drawing to happen). It's always rule of three though for the chibi drawings you can trigger. Some of the drawing you can't trigger though, and you just have to wait for your guy to do them when he interrupts to take control, but the best way to get those seems to just be kinda random about where your clicking or try doing things that are slightly related to the scene you want( such as if you want him to finger or grope Haato but, lick his fingers a bunch by using the mouth icon on them).