Maybe it is a little too narrow for him, but it may just be that, if he's not travelled for a while, he may be persuaded to go in, but the fear of having to travel again on the way back may be too much; I know my horse can be a little bit like that sometimes!
Also, if he doesn't go in it that often that doesn't always help. I know one time my horse hadn't travelled for 6 months and I had to spend 6 hours the night before taking him to the vets to get him accustomed to it again. Try spending half an hour or even less with him around it every day if you have chance, or even a couple of times a week. I used to spend time with my boy, sitting inside the floor of the trailer and eating, whilst letting him have his feed as far up the ramp as he would go. Eventually he came in of his own accord, and I'd just let him stand there in the stall area and give him a haynet. Only once he was completely relaxed would I put the bars up.
Bateson trailers are fab for the airy feeling, so they may be worth investing in, but I know my boy doesn't like Equi-Trek's that much as he tends to like the idea of just being able to walk through. I tend to bend the partition outwards if I can, take the front bar down again and - rather than persuade him to walk into the trailer, persuade him to walk through it. Once he realised he could walk straight out the other side, he was a lot more chilled about going in. Hope this helps; definitely listen to your horse and let him take his time if you can, spending lots of time around / in the trailer with no force definitely helped me