There are two distinct issues here, although they are related. First, let's discuss the reading comprehension. Professionally, I have serious doubts that your son's comprehension is so low. It would be nearly impossible for him to achieve and be recommended for honor classes without having had good comprehension. Some teacher along the way would have noticed something. I find it hard to believe that all of them would just give him A's and B's. I recommend you contact his school counselor and review his reading achievement scores from the school tests. Compare those with the Sylvan scores. Talk with the counselor about any discrepancies. Don't sign a contract with Sylvan just yet.

I believe your son needs testing, but not a reading test. When grades drop so drastically and behavior changes so much, parents need to rule out drug/alcohol usage through a drug assessment by a professional. Contact the school counselor for a possible free assessment. Don't let your son talk you out of it. Emphasize that it is to "rule out" any drug/alcohol abuse.

As for the girl, you have already experienced a typical teen reaction -- you pushed and forbade and he pushed back. You can and should certainly limit his talking on the phone to anyone and how much he goes out. Sometimes a reverse technique helps. You might try telling him that he can invite his girlfriend over on the weekend for dinner with the family or for a movie at your house (while you are there, of course) when he has done all his homework for the week. You can have him bring home a report from his school to verify this. If she truly is the problem, she either won't come, or she will come and everyone can learn a little more about each other and she will cease to be a negative influence.