First off I would rule out any pain related issues, get saddle fit checked, back checked, teeth checked etc. Once all these have been ruled out I would then spend time doing lots of groundwork exercises, lunging, long reining and then when you are ready to get back on make sure there is someone else there with you.
If you have another friend that is able to go on horse back that might be an idea too. You say you have had him just a few weeks, have you moved him away from what he is familiar with? If so he could be very unsettled due to the move. Does he have any others horses there? Either way he could be still settling in to his new surroundings or if he is there as the only horse he may be having confidence issues if he is used to having other horses in the field etc.
I hope some of that is of use to you and if you want to ask questions feel free.