I'm going to make an emergency call to my sons school
Why are you going to make an emergency call?
I'm worried about his health
If your son has health issues then it's good to make sure he's okay in school. Explain to the staff there what he is going through so he can receive the proper care.
He's overweight and I texted him to ask if he's ok and I got no reply.
I'm sure that he's going to be okay, he may just be busy at school. However, if you are that worried then you should really talk to the staff there and explain the situation.