How do I get my child to pay attention/focus better?

I have a four year old and a three month old and this has been an issue before my youngest was born.

He won’t listen to anyone and he can’t focus on paying attention to things when we try to get him to learn his alphabet. We can tell him over and over for a whole 30 minutes-hour that the letter A is.. A. And we’ll come back later and ask him what letter it is and he’ll say R or F or something and then the rest of the letter in the alphabet is not R or F.

Just today we just woken up and he comes up and says good morning to the baby and tells me he has a eyelash in his eye so I sit him on my lap and try to tell him to look up but he moved his entire head so I told him

“You see my finger? Follow my finger when I move it” and he just stared at the wall so I snap him out of wherever he is and say it again.. and this repeats another five times.

I told him that he needs to pay attention if he wants me to help and ask if he’ll listen and he just whines and says no and body slams into the bed. No matter what I do with anything he’ll test me when I say no or stop and not listen, and things he used to know before he just whines about doing it now.

He isn’t in preschool so I’m trying to teach him this myself but he just won’t listen and I’m beginning to think I might have to take him to a behavioralist or something :/ I’m not sure what’s going on but trying to juggle the both of them while this is happening has been draining and depressing me to my max