At what age was your child speaking sentences?

I’m getting a little frustrated with how my bf and his ex are handling this situation but I can’t really put my two cents in because I’m not a parent I don’t know first hand but I don’t know this just doesn’t feel right to me. Their son is 4 and he only knows a couple of words. There are other things that made me feel he may be on the spectrum for autism. I brought it up to him and he got a little defensive. So i dropped it, I LATER brought up “hey we should take your son to see a speech therapist” and he agreed however the place we wanted to take him needs a dr referral. His dr said it’s normal for him not to talk yet and should be fine once he’s in school??? Am I in denial?? Am I wrong for thinking a 4 year old should be able to talk more? -Again I’m not a parent and I don’t know who else to ask hence why I’m asking here.

So anyways I told him recently he should consider signing his son up for preschool that way he gets the hang of things before kindergarten. But he doesn’t want to because…………. His son can’t talk yet and can’t communicate what’s going on there 🤦🏼‍♀️ ( he gets nervous sending his son anywhere around strangers since his son can’t talk) so he thinks by this time next year his son would be good to go for school.

Idk this just makes me nervous and I can’t help but feel his son needs more help with talking but apparently his parents and dr think it’s fine so I’m trying not to really intervene. I guess I want to know do you guys know of any let’s say “late bloomers” a child who didn’t talk till they were older or in school?