i will be getting a trainer out here soon. I don't have a round pen so its a little difficult. He is dead saddle broke though.
Great that you can get a trainer to help you. It's generally far easier & more effective to learn from someone hands-on than on your own with only theory, be that forums, books, vids, etc.
A round pen is not necessary at all(I prefer corners personally). An enclosed, safe area is helpful at times though, but if you don't have that, the easiest & cheapest answer I find is to buy some tread in posts & a bit of electric fence tape. You can make it round or otherwise, as your heart desires!
Considering he's only 4yo & started under saddle at 3.5yo I wouldn't consider him close to 'saddle broke' yet(& certainly not dead - surely he's got a fair few years in him yet??
), and going out & about confidently with a rider is part of it IMO, especially if you want him to be 'saddle broke' as a good trail horse.
So anyway, whether you take him out or leave him alone & take the mare out, I think 'approach & retreat' is the answer for this.