Definitely consult with a doctor, though there are great people and tips here. I have constant sinus issues mostly on my left side. I've found that 12hr sudafed works wonders to clear out my sinuses and keep me diving with zero problems. My old instructor used to take and recommend the 4hr sudafed (the little red sugar coated ones), he called them dive candy. If that doesn't work try moving on to prescription nasal steroids and/or nasal rinses/neti pots. I don't care for the OTC nasal sprays like Afrin, even one dose typically gives me a rebound effect that is ten times worse than the original problem.
And like 101recon mentioned, the more I dive and the more frequently I dive the easier it becomes for me to equalize and the less sinus problems I have. No idea what the correlation there is, but there's something to it (atleast for me).
Oh and, as typical, if your son starts taking anything make sure to take it a few times without diving to understand how it makes him feel. Even something as benign sounding and common as sudafed, as it is known to increase your heart rate and sometimes your breathing rate. I have had no ill side effects from it, but in a kid depending on his age and weight taking a big 12hr sudafed might effect him a lot more. I'd also recommend taking something like that early maybe 2-3 hours. I have two reason for this, one so that if there are any immediate side effects they will wear off (hopefully) by then and if I am doing a night dive and take it right before the dive or really any time during the day after about 5 or 6pm I find I cannot sleep at night.