If the let down is the issue then he will grow into it with time (over the next few weeks) - so don't move to full time EE, make sure you have a couple of suck-feeds a day so he remembers how.
My DS believed that boobs were for sleeping on, so I mostly expressed for the first couple of months. 5 or 6 times a day. By 6 weeks I was expressing over a litre a day...
He eventually got the hang of staying awake and we were able to move on to suck-feeding.

If you have a fast letdown, try feeding with you lying on your side and bub lying on his side on a towel. That way he can just let the excess milk flow out of his mouth without being choked.

Good luck!