My son has hemophilia, but hasn't had a major bleed in about 6 years and takes regular shots. Is it safe for him to play tackle football for his hs?

No. I think you need to speak to your son'e hematologist. As you know hemophilia is graded as mild, moderate, and severe. I don't know the severity of your son's hemophilia. Even if he has mild hemophilia, it is still very risky to have him play tackle football. I have seen a 16 or 17 year old boy who developed severe muscle bleed after playing football first time in his life.
Q for hemotologist. This is a question for his hemotologist. It will depend on what sort of hemophilia he has and how severe it is. If it's bad enough that he has had major bleeding episodes in the past, then the answer will likely be no. Contact sports can lead to serious internal bleeding in folks with bleeding disorders. See his hemotologist for a more definitive answer.