If he likes a french D, he’ll probably like what, in western terms, we call a dogbone or Billy Allen mouthpiece.
http://www.valleyvet.com/ct_detail.html?pgguid=5d79837f-cf87-4f72-a710-ae1e68cf3630
A lot of horses also like what’s called a correction mouthpiece. If yanked on, like anything, if can be severe, but a lot of horses like it because they can hold it in pretty much whatever shape they want and with all the joints, it has a lot of feel, so with good heavy or weighted reins, the horse gets a lot of pre-signal when the reins are picked up, before contact is ever actually made. It can be a very light and kind bit, if you let the horse respond to the pre-signal, instead of just taking ahold of them. I’ve never ridden a horse that wasn’t happy in one. Here is one with a short shank:
http://www.valleyvet.com/ct_detail.html?pgguid=f56a1152-d37b-4bcf-8e19-c61ebeb9243f
I hesitate to include the next bit, because it’s not pretty, or traditional, but every compliant, willing-minded horse I’ve ridden has absolutely loved it. The only horse I’ve used it on that didn’t like it was a hard headed SOB that just loves his 7" shank correction bit, so who knows. It has a very slight gag action, so you’ll be pulling the corners of the mouth up when you take ahold of it and getting more poll pressure than with a standard curb. I’m generally not a fan of gag bits, but for whatever reason, the horses like this one and we’ve used it as a transition from snaffle to curb pressure several times.
http://www.valleyvet.com/ct_detail.html?pgguid=f1b20a8c-4d81-46eb-8dbc-9dd4eab8767c
The links are all to Valley Vet, because they have really good pricing, and it’s the only place I know of that carries the last bit, but please ignore their training advice in the descriptions.
I know you want something restrained in style, but I really like the following one, and it has a hinged Billy Allen type of mouth that your horse would probably like, with some tongue relief.
Just some ideas. Ideally, you’ll have a friend with a bit collection so you can try some out to see what your horse likes before investing in one, or several. Good luck.