Sometimes it's just innate, you can't make him dominant if it's really not his personality and makes him uncomfortable. But I feel your pain.

Making him feel comfortable can help, as could talking out what you want. It could be he's had a lot of bad information/negative feelings about sex, and that's what's holding him back. If he's just shy about sex, then that might just fade over time with comfort.

Giving him some direct guidelines to work with might help - if you request something really specific sexually that would put him in a more dominant position and are really vocal about how much you'd enjoy that, he doesn't have to try and improvise in a situation he doesn't naturally gravitate to and your positive attitude and reassurance will help keep him from worrying that he's being mean or inconsiderate in any way by being dominant.

You can also just straight up say, "I'd like to try some new things in bed, particularly with you in a more dominant role. Can we talk about ways you might be comfortable doing that?" It's usually best to just talk about these things.

It's embarrassing but you just have to tell him, be open about it and don't skirt the topic or he won't understand it