i have a question for moms of toddlers. my son will be three next month and since his birthday is…

i have a question for moms of toddlers. my son will be three next month and since his birthday is late and wont start school for another two years I have started our own school learning at home. Well we learned our colors and he does great every color but red. I have notice that when he really dont know the color he gives me a look and kinda just guesses but with red he is not guessing he honestly thinks its blue. i have held up blue and red beside each other and tried to explain which one is red and which is blue but he just looks confused. I always correct him but after a few times of correcting him he gets mad. This is the only color he does this with any other color he gets wrong he doesnt get mad when i correct him and he learns after about a day of away correcting him. Im starting to think he is color blinded to this color. Shiuld i bring it up at his 3 year check up to his doctor or just put it off as he will learn it later on?