First of all, let me discuss tubes with you...
You claim your son had tubes put in at 7 months. Are you positive they are still there? The reason I ask this, is because tubes are placed by cutting a small hole in the ear drum and the tube placed in it. This allows for easier draining of the fluid that builds up behind the drum(mucus from sinuses is the fluid). They, however are designed in such a way that they fall out as the ear drum hole closes up.
Having said that, there are cases where they fall out and lay in the ear canal, or do not come out at all and can be removed with a simple procedure later. When they fall out, depends entirely on the individual, it can be anywhere from a couple of months to a couple of years later. In my oldest she had them placed at 15 months. One came out but sat in the ear canal and the other never came out...At three she end up needing to have her tonsils/adnoids removed so they took the tubes out at the same time.
Now...Having explained what an ear infection is in the above, Swimmers ear is the water that becomes trapped in the canal by the individual's wax. It usually becomes painful if not removed (dried out, removal of wax, etc) prior to wax sealing around it and thus causing a pocket of standing water infections can brew in.
Now. Swimmer's ear is usually treated with a type of ear drop that dries out the water. Ear infections are usually treated with antibiotics (and sometimes eardrops) due to the fact that the infection is already apparent (hence the redness in the ear canal). Ear drops rarely are effective in the case of ear infections other than to help you clean out excess wax that the infections has caused or any drainage (puss or mucus leaking from the drum itself)...
Swimmers ear can have a discharge, but it's mostly crusted wax from the ear canal that the water is pushing forward.
The best way to prevent swimmer's ear is to have your child wear ear plugs (the silicon one's are highly effective and easy to follow directions on use)when they are swimming.
If it is an ear infection it could be complicated by swimmer's ear or just taking longer to heal if you are allowing your child to swim while getting over the infection.
Also, there are several different antibiotics out there used to treat this. You child may have used one enough times that it is no longer effective in your son, or the infections itself can be resistant to the antibiotic.
When my odlest was a baby she had sooo many ear infections in a row that they no longer responded to the antibiotics and her ear drums began bursting from the pressure.
THIS is how an ear infection can cause hearing loss. The bursting of the ear drum from the pressure created. If there is scarring ont he drum from this it will cause hearing loss.
However, if your child is in need of tubes it may be because his ears have an issue appropraitely draining the fluid build up. This would make everything they hear sound like it does when you are under water. (this specific problem cause speech delays in my now 4 yo son - at 2 he was 1 year behind in speech because he had constant "fluid in his ear" and could not hear sounds like you and I could. He was given tubes and placed in speech therapy, he caught up to his peers in speech in under 1 year with therapy)
So...In conclusion, I hope I have answered your request and I would highly suggest having your son seen by a doctor to determine 1) are the tubes still in his ears, 2) is it an ear infection for sure or maybe is is swimmer's ear, and if it is the infection, 3) does he need another set of tubes.
After a certain age his ear canal should have developed to a size sufficient with fluid drainage and you should not see very many if any ear infections. However, some canals do not get very big and it remains an issue and he made need a new set of tubes.
If your son does still have his tubes he should never be in the water without earplugs as it aggravates the fluid drainage problem.
Good Luck... ;-)