My 2 year old son has a really bad sinus infection, is there anything over-the-counter that works great?

Not sure: It is hard to judge exactly by the description given on what's going on, but if he is running a fever, throwing up or not drinking/eating and feeling slow/lethargic, he must be seen. This may be more than just a sinus infection. If no above symptoms, i'd recommend keeping a humidifer in his room and dripping his nose with saline solution. Occasionally Benadryl (diphenhydramine) may be helpful.

Not really: True sinus infections are rare in a 2 year old as their sinuses are not highly developed. He may have a upper respiratory infection or something else. Many otc meds carry risk of overdose for that small a child. Saline drops and tylenol/motrin (per directions) may be helpful. If his symptoms are indeed "really bad" i would bring him to his pediatrician. Best wishes.

NAC + garlic pills: You can do nac which is mucolytic and garlic pills which works as antimicrobial or you can use adp pills which is oil of oregano - this will be spicy taste easier to do the garlic caps. Have then take the caps with water and bend their head so caps float up.

Saline: Saline drops or rinse will help clear the nasal congestion. A sinus infection in a two year old is difficult to diagnose and you should bring your child to your pediatrician if you suspect this.

Sinus infection?: 2 year olds frequently get colds usually not bacterial sinusitis. Bacterial sinusitis is way over-diagnosed. Children in daycare often get 10-12 colds per year. The symptoms between cold and bacterial sinusitis are similar. If symptoms last more than 7-10 days then a bacterial sinusitis or allergy may be involved. Try nasal suction with bulb syringe. See ENT if symptoms persist.