I agree with Mackai, using the paste will not only help harden your horses hooves it also helps (very slowly) if your horse has holes, cracks or extreme indents from shoes. My friend has been using it on her horse and she's slowly getting a brilliant result x
My horse also has really soft feet and he kept losing shoes in the pastures so my farrier suggested hot shoes for him and they have stayed on sooo much better! We also have him on biotin supplements to help his feet!
My horses have the same problems. My farrier recommended this to me and it works really well!! Mix some sugar and iodine together into a paste and paint it along the soles of their feet and the hoof wall. You may have to do this a few times if your horse's feet are really soft but it'll help harden in really fast. The iodine dries the crap out of their feet. Also while you're doing that, if you still want to go hacking and your horse's feet still haven't hardened fully, try some easy boots those work wonders! It's basically a substitute for shoes without the big $$$. Hope this helps good luck!!
I personally would get him some hoof boots/trail boots as you stated. The horses hooves won't harden until the hoof has 100% grown out, this can take up to a year. To help this, adding biotin to the diet will help & making sure your horse is on a high fibre, low sugar & starch diet is key.