Whether to opt for IB or AP may have more to do with what he wants to study at university than anything else. The IB is recognized by many American universities as being nearly equivalent to the first year of university. AP gets you credit in specific subjects. (Unless things have changed lots in the years I've been away from the US.) AP courses and test results are often the most useful when applying for science programs in high powered universities - math, chemistry, physics, etc.

As far as credit toward HS graduation at summer school - sure, it's very possible, though many school districts offer summer school primarily for those students who need extra help to keep up to their grade level in certain subjects.

Another option is to seek out some nearby universities, which often run "summer universities" or other special programs for high school aged students. Though they don't necessarily count credit toward high school graduation, completion of such programs looks good on college applications (especially if your son is applying to the universities where the programs were offered) and can be a means of familiarizing him with the "style" of teaching in the US.

But if you're planning on having him attend an American college or university, remember that admission is based on being "well-rounded" and often extra-curricular activities are considered important. These can be anything from music and art to sports, student government or an after-school job (paid or volunteer).
Cheers,
Bev