Please help me understand - autism and time

my son is 5, has autism, and attends a mainstream school. One of his difficulties is that I find he cannot wait for anything. If he asks for something, and I tell him ‘in five minutes’ he will often cry and shout it takes too long. I have tried linking with our routine ‘eg after your toast’ or ‘after we have lunch’ but he will still get upset  I darent mention his birthday as he will cry on occasion saying it’s too long away!! He needs to has what he wants immediately... even when making cupcakes he gets upset about the 12 minute baking time. Why is it he struggles with ‘time’ and how can I help him not get upset... he doesn’t seem to learn that in life, we have to wait for things, especially good things!!  I just want to understand how he sees things