When you said you were fooling around with him, do you mean that you had sexual intercourse (either vaginal or anal) with him, without using a condom? Intercourse is considered a high risk activity for contracting HIV. If you had oral sex with him (like you going down on him or vice versa), it is considered a low risk, and petting or necking presents only a theoretical risk (basically no risk at all). Of course, all this is assuming he is HIV+.
If he's not HIV+, then obviously you have nothing to worry about. However, you don't have absolute proof of this, and obviously it's worrying you a lot. I would suggest that, in order to alleviate your worry, go to an STD clinic yourself, and have an HIV Antibody test done. It's the only way to know for sure if you are HIV+ or not. Some people experience flu-like symptoms about 4-6 weeks after being infected, but most people show no symptoms of HIV disease for several years after infection, so it's impossible to know without being tested.
Now for the bad news. In about 97% of cases, it takes up to 3 months for HIV antibodies to be present in the body, and up to 6 months in the remaining 3% of cases. Since the HIV test is based on the presence of these antibodies, you will need wait until 3 months after the last time you had unprotected sex before getting the test done. If you go to the clinic before the 3 months is up, your test may be a false negative, in other words, you may test negative for HIV but still have it. So you'll need to wait at least 3 months to be absolutely sure. And during the 3 month waiting period, it's important to have
absolutely no unprotected sex. If it turns out you are HIV+, you could be infecting someone else without knowing it.
There are a couple of other HIV tests, one is called the p24 antigen test, and the other is called a viral load test. However, these are not usually used to diagnose HIV. The p24 antigen test is most commonly used to screen donated blood, and the viral load test is used to measure viral load in someone who is already HIV+. If you are really keen on having the test done before the end of the 3 month waiting period, you can inquire at the clinic about these tests, but it most cases, they are very expensive and not available for use as a diagnostic tool. Be very wary of any website that claims it can sell you one of these tests!
If in the meantime you are very upset and depressed, I would suggest that you contact your local HIV/AIDS organization. Usually they're listed in the phone book under "HIV" or "AIDS". Or you could ask to speak to someone at your local STD clinic. Most of these organizations have counselors and nurses you can talk to, who will listen and try to alleviate your fears. They are there for you and happy to talk to you even if you turn out to be negative for HIV. I used to work for an AIDS organization as a peer counselor, and they are really awesome.
Normally in a medical situation I advise people to see their doctor, but since I don't know where you live, I'm not going to advise that, because, for example, where I live here in Canada, if you go to the doctor and test positive for HIV, the doctor is required by law to report you to the government health agency. Personally I consider this to be a violation of human rights. So, that's why I suggest the clinic, because that way you can have completely anonymous testing.
In any event, please go talk to someone at an HIV/AIDS agency or a clinic. I think the only way you will ever have peace of mind is to get tested. Good luck, and please come back to let us know how you're doing!
Here are also a couple of reputable websites where you can learn more about HIV/AIDS (there are a lot of bad websites out there, so please try to stick with these two!):
http://www.avert.org
http://www.thebody.org