Is this a major red flag?

My boyfriend of nearly six months has a strained relationship with his mom. They mostly fight because his mom and him are very different and I get that.
But what gives me red flags is that he's constantly mentioned how much He hates her and whenever they fight he wants to slap her and he mentions this every time his mom comes up in conversation... Which sends warning signals in my head.
He's a nice guy as far as I know but I'm not sure If this is a red flag or not?
Is this a major red flag?
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