Hi,
could you tell us how old your son is? From your post I am thinking he is about 4 months but that could be incorrect.
If he took a bottle just 20 days ago, then I think that there might be something else causing his refusal. I don't think he would forget. Could it be that he doesn't like the formula now, have you tried expressed breast milk in a bottle? If not, try changing the nipples / bottle etc. Maybe he is getting frustrated if the flow is too fast/slow.
My DS did refuse the bottle but he was exclusively breast-fed - we tried and tried and no luck. We ended up just giving him a sippy cup at around 6/7 months, and he still didn't take a bottle until he was 11 months! And then he refused to give it up