I am wondering why I wouldn't be getting the entire $2,500 American Opportunity Credit for my son in his 2nd year of college? instead, it's giving me $757.

Locate Form 8863 in your documents to see how the credits are calculated. You may need to pay for the tax preparation first before being able to view the the forms.

What's on his 1098-T.  Did you take a distribution from a 529 Plan?  What other info can you provide?