Because of the tubes, you could first go to the audiolgist and have his hearing tested, since you don't think he can hear his errors. If he is intellegible, then you may have time to wait it out. I would suggest that you know for sure if you need therapy or not, and I would not set out to DIY a true articulation disorder if he has one, instead of articulation errors that are still within the typical limits of his age.
Write to your school district and ask for a speech evaluation because you susepect that your son has a speech disorder. I would not expect services based on mild articulation errors, but you will get some scores in the evaluation data that will give you some idea of where he is in the bell curve because the evaluation instruments will give you numbers based on his performance compared to children of his exact age, and a large number of them. The number should be based on the number 100, and more than that is above the average, and less than that is below. You should also get some sense from this evaluation if his difficulty is just articulation, or if he also has trouble with langague, and his IQ will be screened, so you will get a sense of processing concern. You should pay careful attention to the numbers, more than a single area of concern should suggest to you that this is a difficulty that he will not just out grow, even if the school district decideds to turn you away for therapy. The school's bar is very low, they only have to make him "funcitonal" in the classroom, and while many districts do more, they certainly do not have to. If he gets therapy from the school, I would highly recomend that you also get a private evaluation becasue you need to be certain that the school is providing all that he needs, and since they are not required to do that, it is a safe bet that he will need more privately, which is just the nature of public serivces.
Intellegiblity is the issue usually. It is very odd that you say that strangers don't notice it. How do you know that? Maybe they are just not saying anything? Just a thought to toss out there, I am astounded sometimes how many people will wait until Mom and child are gone to say "did you hear that?" Just want to caution you that you may notice it less, not more, than people who don't know him (or you) well, and don't want to risk being rude.
One thing to think about too, a private speech evaluation is usually covered by insurance so call and check. They are not very expensive, even if not covered, and a speech evaluation is a win-win. You either walk out the door knowing that you are going to get your child help that they really need, or you know for certain that they are fine and don't need help. Development is just far, far too important to leave to chance, and chance. Catch it early, even if the chance is tiny.
M.