My stepdaughter had two sets of grommets in her ears(shes now 14) they  fell out of both ears the 1st time. ent said they did'nt need replacing. My stepdaughter was very loud and disruptive when she started school,and would turn the tv up full blast. My wife took her back to the ent and he said straight away she was lipreading,she had virtually no hearing in one ear,due to fluid. She had another set of grommets put in which lasted about 18 months. My wife took her off all dairy (it made a huge difference to the fluid build up) but she has a bit of permanent hearing loss in one ear. My advice would be to do the grommets sooner rather than later.

If your child is prone to recurrent ear infections it is well worth having the grommit replaced. This prevents any permanent damage being done to the ear drum which results in permanent hearing loss. Grommits do come out on a regular basis as the child grows, but once their eustacian tubes straighten out at age 5 -7 they shouldn't get as many infections, thus making the need for gronmmits less necessary. Speak to the ENT specialist if you are concerned but as it is a relatively minor procedure, the benefits far outweigh the risks. If infection builds behind the eardrum and the eardrum bursts, then the eardrum becomes scarred. If this happens on a regular basis, hearing is impaired. Grommits prevent this build up of pressure behind the ear drum and consequent damage to the eardrum occuring.

My brother's friend had the tubes and they far out weight the cons.  If it is helping your son with preventing ear infections and allows proper drainage of the fluid, I say you should do it again, since children grow and they need to be resized as they get older, some cases they are ok witht he first ones.  But talk to the doctor and see if it is necessary and how you feel about it all.  Good luck!