We have tried to give him EBM about half a dozen times though and although he did drink it once, more recently he has a huge screaming hissy fit if dh or I try to put a bottle in his mouth.
He usually goes to bed by around 7.30 so I can go out after this, but it was on a couple of occasions when I have gone out and ds has woken up again that DH has got a bottle out and the screaming has ensued (for about 2-3 hours until just before I got home!!)
The thing is, I don't really want to go through the process of trying again and again and upsetting all involved, but I think it would be useful for him to be ok with a bottle. But I don't want to express for the sake of it (to get him ok with a bottle), for the very occasional time where it might be useful.
I too tried to get DD used to a bottle just in case I needed to be away from her. From 6 weeks she took expressed BM without much problem, but after a gap of a few weeks she refused and never took one again. I'm sorry to be pesimistic, but you may find that 20 weeks is just too late to start. It's possible that he won't accept a bottle until he's old enough to take a sippy cup/straw/cup. Maybe someone else will have more constructive advice.