I've only just read this thread now... firstly, I'm so sorry to hear that this has happened! But I'm really glad that you've turned here for advice!
First thing first... good idea with doing the ground work. It's gonna help you both with your bonding. However don't take TOO long. You do need to get back into things (as long as he's all fine and not sore anywhere!)... the longer you avoid the situation, the harder it may become to address (or he may be a gem). I personally wouldnt even dream of putting my foot in the stirrup to begin with and I dont think anyone mentioned not to do this earlier so here's my idea!
Make sure you have someone to hold your guy and get yourself a mounting block/bucket/anything you can use to get some height and have that on standby... now once you've played around with your saddle on your guys back (ie, flapping the stirrups, bounce on the ground beside him, rattle things, 'slap' his back with your hand (not hard, but just like a tap) - really desensitise Major! Once he's standing still for that, repeat all of that but up on your bucket/block. bounce on it... really act like he's new to the saddle... cos he has to know that things may go wrong... and its okay for him to spook... but its not okay to remain scared forever, and that you're going to take a step back and readdress everything.
Then INSTEAD of mounting or putting weight in the stirrup, I suggest leaning over his back. He's not going to anticipate that... and by the sounds of things, it's when you put your foot in the stirrup he thinks somethings gonna snap. So that's why he's not allowing you to mount... but if you take the stirrup out of the equation and mount, he's gonna hopefully go "oh. it didnt snap"... go from leaning over his back, to leaning over and then swinging your leg and mount that way. once you're sitting on him... praise... get off... repeat. make sure you do it three times, then give him some love and let him go.
Once he's really happy with all that, start using the stirrup to hoist you over his back and lean over... and then get up... you may be able to miss this step if he accepts you back up on him and realises the danger of it snapping is gone. Finally after he's feeling happy about that, its time to say "hey, im gonna mount you properly now!" and do so. Make a big point to not rush him.. but to not baby him either. I know he's scared and obviously don't put yourself at risk... but don't drag this out over weeks. He needs to know that it's okay to be upset over something, but that life does go on!
I hope this has helped!
♥ Seoul Searchin' for the Lovebug ♥