Ah. So he's one of those. Honestly, I will never understand the people who legitimately cannot distinguish someone being a kind person for attraction. And especially those who take it a step further by thinking they're in a relationship as well. Like, that is lock them in the psych ward and toss the key level delusion
And I pray that you never do.
However, please allow me the chance to at least try to evaluate his behavior. From what I see, he's obviously playing the "Because you were nice to me" card which kickstarted his attraction. However, given his background as the son of someone wealthy and influential, he's probably too used to getting his way, so when he sees Shizuka going out with someone that ain't him, that's when the jealousy starts.
So basically, not the best writing, but it's simple enough that for some, it works.