My grandson does have speech difficulties and at 4 !/2 still has difficulty being understood. He has always just came and got someone when he needed or wanted help. He still comes to get me when he has to go potty tho he doesn't with his mother.
I'm guessing that if he were aware of his need to go potty he'd find some way to get your attention. Therefore he probably just hasn't learned yet to connect his body's feeling with his need to go potty. 27 months is early for a boy to be fully trained. That is why it helps to take him to the potty every so often. It needn't be every hour especially if you can watch and see how often he does go. Then work on a schedule that is most likely to catch him ahead of that time.
My grandson has always sat directly on the toilet. If you face your son towards the back he will feel more secure. Put some toys on the tank for him to play with.
When you say squats do you mean he keeps his feet on the seat and squats down? That does seem more scary to me than sitting down. Perhaps if you practiced letting him flush the toilet and talk about his fears you could reassure him and he'd realize that nothing happens to him. Saying bye, bye poop often helps.
My grandson also usually stood up to pee from the start. His Daddy showed him how. That does mean that you'll most likely need to be quick about redirecting the flow some of the time.