The thing is, this probably isn't just anxiety, it's anxiety plus habit. After a few weeks like this, the child conditions themselves - the morning becomes associated with stress, and in the end it doesn't even matter if everything goes fine at school. Her brain is already expecting the morning to be difficult, and her body responds with stomach pain and watery eyes. The risk is you keep negotiating, trying to convince him or giving in once in a while (like letting him stay home one day because "he's too anxious") - that reinforces the idea that anxiety is a valid reason not to go.
Change the dynamic. On the weekend or a late afternoon, sit down with him and talk naturally about what he's feeling, without pressure - not "why are you scared?" like an interrogation, more "I've noticed mornings have been hard, tell me what's going on". Sometimes something comes out (a kid who stresses him out, fear of not being able to do an assignment, anything). But then in the morning, ignore the complaint - doesn't mean being harsh, I mean say it clearly: "I understand you feel bad, let's get in the shower now". Keep the routine calm but firm, no drama. And as soon as he gets into school - rewards at the end of the week if he can get in without a fuss, something simple (extra time with a game he likes, baking cookies together). This isn't ignoring his feelings, it's teaching him to move forward despite the discomfort, which is a skill he really needs.