This is only my opinion, but based on experience. The economics of buying a frame and fork and all the components don't equal out to a very cost effective plan. However, if cost is no object and the fun of building up a frame and fork is something to look forward to, go for it. Enlist your grandson to help you out.
Not knowing if he'll embrace touring, and not knowing your budget, I suggest starting out by buying a medium to high quality older mountain bike, adding a rack and appropriate tires and with your grandsons help, repacking all the bearings and cleaning everything up.
Either way, you're a lucky grandfather to be able to take on something as great as touring with your grandson!!