Having trouble cumming with my partner

Hello..I tried looking everywhere for answers and questions for this and couldn’t find a single thing about it- coming here was my last resort but anyways.. I’m having a hard time cumming with my partner- especially with oral, he’ll be eating me out which he does a good job at it and I feel very comfortable with him. But my body doesn’t want to cum, It like clenches in a way? Like it’s holds everything back and I don’t get that sensation of cumming.

Very hard feeling to explain, I want to cum, I feel comfortable and relaxed with my partner and he’s really good at what he does like I said and I’m not worried about anyone walking in on us.

if someone has any answers please help- if you need more information I’ll say. Thank you if you can help- much appreciated.