Bottom too tight or not comfortable

I have a FWB who bottoms for me. However, when we first started he would just suck me off, and it took several times of us getting together before he bottomed for me.

The thing is he is just too tight. We do go slow, have lube, but says it hurts too much. Im not big nor thick. The only position he is ok with is riding my cock, either reverse cowgurl or regular.

im guessing that he needs a plug to stretch out a bit before we hook up? Im not a bottom and dont know about getting fucked.

He is married to a female, so we are discrete about our hook ups.

Any advice on how we can get him bottom comfortably?