it depends on his english skills. all DR school paperwork is transferable if processed correctly (i don't know what the reverse process is - only how to process US paper work for the DR, sorry). He'll probably need to take the TOEFL exam as well to see if he needs remedial English language classes as well.
I know that USF offers a program for immigrant, non-english speakers as a conditional acceptance to study there. But there is also the County Community College (whose name is escaping me now) that is cheaper and might be a better choice for his first classes.
As far as what he's already done here in the DR - most of the schools are recognized. However, not all of the credits will be accepted and those that will might just be counted as "electives" instead of what the original class was. That depends on what his major is (not where he studies - I've seen unis in the states take UTESA credits as they are far more often than the PUCMM... and in tourism and education classes OyM transfers the best) and how many semesters he's studied. A lot of American universities won't accept credits past 65 (which is approximately 4 American semesters worth of classes).