How do I get him to learn?

I try and try and try to teach him things and he just cannot focus! Today we are going over days of the week and I told him ( and he repeated 5 times) that today is Wednesday and then when I ask him what day it is he says "I can't remember". What?! It has been 5 seconds! Sitting at the table, we will do little counting exercises and he ALWAYS skips numbers and says he doesn't know, but then when counting the days of the month, he counted to 17 with 1 flaw. I am at a loss. I feel like he should know so much more by now, but is just unable to focus and learn it. I worry about him when he becomes school age and whether he will have the same difficulties as I did concentrating. I know he is only 3.5 and I don't expect him to know multiplication, but I expect him to be able to sit for more than 30 seconds and recite the days of the week by himself after learning it in preschool and being reiterated by me at home. I don't want him to be so far behind everyone else ... that is the worst thing that could happen for someone like Ethan who loves to be at everyone elses level. Am I expecting too much? He will be 4 in October and we have to trick him to find out what he knows!