First & foremost, get yourself educated about what you're doing & the hows & why's of what's happening to his feet. Hoofrehab.com is one great source to begin with. Learn the pros & cons of different principles, approaches, etc, so you don't just have to take someone's word for whatever. There is also many more important factors than 'transitioning' gradually. Most of them - like diet & environment - are your responsibility as owner.
I find it curious that the trimmer said your horse *will* be 100% sound in a year, let alone better. Either with much more information than we have, he knows your horse's feet are already very good and his management is optimum too, that he lives on rocky ground, exercises a lot, etc... or he's overly optimistic or likes to encourage his owners to be. As said above, there's a lot more to it than just pulling shoes & 'transitioning' and due to any of those factors, a horse may or may not be 100% in all terrain ever, let alone 6 months(another reason for doing your homework). Not meaning to sound negative, just realistic. Not all horses can cope bare in all they're asked to, period. So saying, I do believe that keeping horses shoeless is generally the far better option for them. Just that many will need boots or such for some of their work.
While giving the horse a substantial amount of time(but starting gradually) on gravel is a good idea if you want them to get conditioned to it, there's gravel & there's gravel(for eg. 4" deep pea gravel is generally comfortable for hooves in any state), and also it depends on the state of their feet as to whether they need to grow stronger before even starting to work on conditioning. If for eg the horse is thin soled, there's little protection against stone bruises. If they're weak heeled, they can get bruises in the frog, sensitive heels can cause them to 'tippy toe' and bruise toes, set up excess leverage on toe walls & other bad mechanics that can lead to further injury. Therefore it's important to keep them well trimmed and comfortable, for optimum hoof function so they can exercise their feet properly & build strength & health enough so they may *then* start 'transitioning'.
At whatever stage, when taking your horse bare over terrain you're not sure if he's up to, err on the side of caution, allow him to pick his way and always listen to him & quit doing what you're doing if he's not comfortable. Don't wait for real difficulties which could signify bad bruising/abscesses. If you can't/don't want to take it at your horse's pace, just put boots on.
Your trimmer is right that *generally* horses do tend to do fine without back boots. However, there are lots of ifs & buts & exceptions, of which it seems your horse is one. Sounds like 4 boots are the way to go, and that your horse may not be even up to driveway jaunts barefoot either yet. If your drive has deep piles of pea gravel, or you can add some areas of this to his paddock hangout areas tho, this is *generally* fine even for tender feet.