What's the best way to handle an overstepping boyfriend (when it comes to your kids)?

Im trying to tread cautiously on this subject with my boyfriend. After all, he stepped into a pre-made family when he did not have to and it has been stressful on him; especially where he has no biological children himself.

So, it's been almost 3 years since we have been together. Within the first year my children's father became MIA and stopped seeing the kids all but twice a year; and that is ONLY when his mother asks for them. He doesn't pay child support so I am 100% financially supporting my household. This is a problem all in itself and we are trying to get court and all that established but due to Covid, it's rather difficult.

Now, my boyfriend treats my kids great. he's always involving them and taking them places, buying them little things etc. The ONLY issue I have is that he oversteps me. For instance, if my kids ask me something like "Can I listen to music on the tv?" I will say yes, help them turn it on, etc. He will walk by and flip the tv off and tell them to go listen to music in their room (he hates their taste in music).

Or the other day, we brought the kids to a water park. We had been there from 10am to 5pm. Around 445pm I told the kids to go down the slide one more time because we were leaving to beat traffic. My boyfriend then said "No, it's fine. Go play and go down a few more times." So, I spoke my piece and told him that I had already given direct orders, etc. He became visibly irritated and gathered up the kids but didn't say much after. However, my kids then thought I was an asshole because I made them leave after my boyfriend said they could stay.

Its little things like this. Just minor over-stepping that is starting to drive me insane. And yes, I've tried talking to him and he says "Well I'm basically their dad now so I should have say too." Which, I get it and I dont want him to think he's doing anything wrong but I dont want to be overstepped either.
What's the best way to handle an overstepping boyfriend (when it comes to your kids)?
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