Having trouble getting my soon to be 3 year old to verbalize his needs?

My son will be 3 at the beginning of june and he still prefers baby babble over worded sentences. He has good days and bad days. Sometimes he talks a lot and sometimes he acts like he doesnt know a word of english nor can he UNDERSTAND a word of english. its like he talks when he wants to and plays stupid when he doesnt want to. When he wants a doughnut or something, he whines for it, despite my many efforts to try to tell him when hes ready to say "can i have a doughnut PLEASE?" then I will give him what he wants. I'll walk away and ignore his whines but he still acts as if he doesnt understand my request to use words to get what he wants. I think the fact that he is almost 3 and still not talkin llike most 3 year olds is a big red flag. He is an only child, we dont know many people with children around his age, the ones we do know are younger and we notice he mimics a lot of the younger kids behavior which hinders his development. We dont have him in daycare because its too expensive and Illinois has cut state funded preschool programs, so now u have to either p a screening saying your child is under developed and NEEDS daycare, otherwise they have to wait until they are 4 or 5 to start school. I think his speech problem would qualify him for an earlier education but i was just wondering if there are other parents who have the same problem or if anyone knows how i can solve this problem at home too. What am i doing wrong?