Shaleeena, the word immersion comes to mind. I had the reverse problem with my youngest. Coming to the U.S. she spoke little English and it was a very difficult first 4-6 months. I had patience and we were lucky she had great teachers at school or it would have been a disaster. She was immersed at school and had no choice but to learn. Luckily we have a great ESOL program here and that is an enormous help. Now she speaks better English than I do but now she is a reluctant Spanish speaker even thought it's her native language. When she spends a few weeks or more with Grandma she is back on the bike so to speak.
You have to be able to strike a balance and I would suggest he have a 2-3 month visit to an Aunt or Grandma in the U.S. if possible. It is not easy for the child and mine had a tough time but in the end it's worth it. The accent is the least of your worries.
Well, of course they have an opportunity to come to the States during the summer. Last summer they did not want to. They spend most of their time in the campo during the summer and trying to convince 3 kids that going to Grandma's house and being stuck watching TV until she takes them somewhere rather than running around in the campo with 50 other kids eating Mangoes from the tree and Pica Pollo from El Chino is very hard and I understand. They study all year so that is why I don't force them to the States during the summer unless they want to (which they don't).
Bertania (my nanny/housekeeper) is like Grandma to them (they call her mam? and me they call mami) and her family is like my family with scores of tios, tias, primos, primas, etc. In my actual mother's house its just her and her husband, my brothers are in a different state. So for 3 kids to spend their summer locked up, occasionally going to Wal-Mart or McDonald's has no comparison to being in the Campo with a family of 25.
But I will let go of the accent worries....if he has it, he has it. My biggest goal is for him to reach 18 speaking, reading, and writing English as well as his Spanish.
SHALENA