Please make sure you are giving only 2 grams a bute per dosing and no more than 4 grams in a 24 hour period. I am assuming the 20 grams you stated was a mistype.

Navicular can be very managable. I had a horse (we lost him at age 30 in 2006) who had navicular, he wore steel eggbars and did beautifully as long as he was done every 4-6 weeks If we let him go longer than 6 weeks he would begin to get sore. We continued to ride him and if I knew I was going to use him more than normal I would give him 2 grams of bute the night before, the morning and evening of the ride and then the next morning, just to head off any extra inflammation. I have never personally used the Isoxyprine but have had many clients use it and some say it made a difference and some said they don't think it did. Some studies say yes there is a benefit and other say there is not really. I think it depends on the horse probably.

I think for now keeping shoes on him would be best. Until you get him comfortable. The most important thing is that the shoe is plenty big enough, with lots of heal expansion. I would either get a recommendation of a farrier from your vet or have your farrier speak with your vet and go over the xrays and shoeing perscription. It is important with navicular horses that you keep them on a strick shoeing schedule.
My girlfriend just recently had a brood mare of her diagnosed and it took about 3-4 months to figure out what was best for her foot and what shoe would keep he sound the longest. She is trimmed every 4 weeks and is wearing a regular show with a ton of heel expansion.

There are a lot of options out there for navicular horses, it is just finding the one that works best for your horse.

General rule for shoes. from the third nail hole to the end of the shoe you should be able to put a nickel on its edge and role it to the end of the shoe. Then if you are looking at the foot from the side, you should be able to draw a striaght line down from the heel bulbs to the ground and the shoe should extend back to that point. This will allow for plenty of heel support and foot expansion. This goes for horses who where regular shoes to special shoes. Most farriers don't want the shoe to stick out that far in fear of the horse ripping off the shoe. I have only had a horse do this a few times over the years. If you are worried about them pulling shoes, then put bell boots on. Ones that will go all the way over the hoof and shoe. The good old fashioned rubber ones that you have to pull on work the best.

The only other thing I would suggest is bedding the horse on 6-8 inches of sand, wether the horse is shod or unshod. The sand will give the horse the support it needs at all times.

Good luck and hope this helps.