You for sure have hinted and you could simply come back with a I tried but you weren't ready to hear the information. Now that we know for sure we can proceed with a plan.
For now write down your observations and areas where you notice he is falling behind the group. That info will be of benefit. Even knowing when he finally started doing a task helps as it indicates how far delayed the child is.
I have one in care now too that we are monitoring. She will be 2 in Oct. At her 18 month check up the pediatrician was concerned enough to reschedule a visit for 3 months later (will be in the next couple weeks). She was concerned child was not talking much, seemed to not care when spoken to as in interact with doctor, could not point to body parts nor point out things in a book, make animal noises if asked, etc. For your boy it may be that the doctor is not very thorough either and that won't help at all.
If you can find subtle ways to show them that the others that are younger than him are doing things that he still isn't that should at least give them the info to be thinking even if it is to just make excuses why he isn't but answers to the I tried to tell you concept. Mention things casually like he didn't get very much time to do a particular activity because he didn't respond to my requests to come nor did he follow the other children to the kitchen area so by the time I got XX and YY who did come ( name two younger children) set up and then had to go get him to escort him personally to the kitchen he missed out on most of the activity. Down the road it should get them thinking or they will be able to look back and then you can put it in a nutshell saying you tried to point out areas of concern based on the fact younger children were passing him by developmentally but since you seemed to be the only one concerned you had no choice but to let it be.