I know exactly how you feel as I went through the exact same thing with my IL's and DH !!
At 2, it was picked up at pre-school that my DS1 was not using imaginitive play & should be saying more for his age (he could only say mama, dada, nana & bubba). My DH wanted to bury his head in the sand & not accept that we needed to get him checked out. My MIL kept telling me that he was "just a lazy boy", and went over and over the same story about a distant relative who also couldn't speak at the same age....it got so boring !!!
Anyway, I had him checked out & the doc wasn't overly worried, told me to come back in 6 months if he still wasn't really talking. I took him back after 6 months & was then referred to a Paediatrician.
Whenever I mentioned the possibility of ASD, my DH & IL's were very quick to tell me I was worrying over nothing, there was no way he was Autistic because he looked us in the eyes, was affectionate, could count etc etc.
In the end, I decided to follow my maternal instincts & do what I felt was best. He was diagnosed with ASD aged 3 (he is now 5.8).
Even after the diagnosis, they still told me that there must be a mistake (cos obviously they knew best of course!).
It has taken 2 years for them all to finally accept the fact that DS1 is different and does have different needs etc.
All I can say to you is, do what you feel is best. You're his mother & probably spend the most time with him out of everyone. It's hard work, but they'll come round in the end. Plus, if you start the process now of trying to get a dx, you can get support sooner rather than later for your son which is so important.
If I can help with anything else, please let me know.