Hi Angle, my youngest had tubes put in when she was around 18 months old (i think) She has down syndrome and very narrow ear canals. I think alot would depend on your ent/surgeon, we did go into the surgery knowing that he might 'not' be able to get them in place. He put the ones that stay in for a few years and they came out when she was around 6 or 7. She does have a hole in her left ear drum which is still there now and she'll be 17 this fall. They advised not repairing it because if she were to have problems that would cause pressure in the ear (allergies/colds etc) before it completely healed we'd end up with a bigger problem. Since she gets no infections despite swimming and washing hair we don't do anything.
I don't get why your sons dr. wanted to wait til your son was 5, unless your son was deaf? One of the reasons tubes are placed is to the child can hear better, the fluid muffles sound so hearing is effected, if there's infection tubes will usually help with that, tho as you said sometimes it doesn't, it's why I wonder why your dr. would put the procedure off if your child is hearing.
I know of a boy with down syndrome who would have the ones that only lasted about 6 months before falling out and he had narrow ear canals but he became very hmmmm crabby and would hit and stuff when he got fluid build up, took a couple times and his mom finally figured out that the first thing to check when he really started hitting alot was the ears. Narrow ear canals if normal for children with down syndrome and they get ear tubes for fluid build-up when very young.