Some points to consider when fitting a bike to a very big man...
Road bike frames are shorter in the top tube than MTB frames, to allow for the extra reach to the hoods (eg in my size, road bikes tend to have ~550mm ETT, MTB tend to have ~600mm). So, the longest off-the-hook frames are likely to be the biggest MTB frames, and they could be stretched even further by putting drop bars on them.
Frame height is probably less of a concern, because bikes are somewhat telescopic in that direction. Long seatposts exist, and most forks come with very long steerers (which are then cut down to suit). If you get an uncut fork, an ungodly number of spacers, and a steep up-angle stem, you should be able to get the bars quite high.
On the surface, a XL sized 29er MTB seems like a good starting point. That will be a long cockpit, as above, and will be made of comparably strong parts. Except that a half-decent MTB is almost certain to have suspension forks, which are quite unnecessary for the road, and are likely to be factory tuned for a lighter, more typical rider weight (one model of fork fits all sizes of frame). Also, MTB cranks tend to be wider spaced than road cranks, to clear wider chainstays... and MTB-sized chainrings aren't really appropriate for road/path use (once he gets a bit of fitness up he might find he's spinning out a 44T MTB big ring quite easily).
So... I'd be looking at as solidly built flat-bar road bike as is available in such a big size, with a view to converting to drop bars if needed for the extra reach. And make sure nobody dares to cut that steerer tube.
Disc brakes if you can get them - because they work, for starters, but also because that gives the option of any number of sturdy off-the-hook 29er MTB wheelsets if he proves too heavy for more road-like wheels.