The main problem with the Windows tablet is that whilst it’s good working on the Windows Metro screen, most programs drop back to ‘old’ Windows which really isn’t easy to use with a touch screen so needs some kind of mouse. I guess in time this will change as more programmes/apps are designed to run in Metro.
If he wants a small, portable Windows machine to sit alongside an existing Windows PC then a Windows tablet makes a lot of sense, as long as it comes with a keyboard with an integrated touchpad.
If it’s going to be his main machine to use in most scenarios then I’d go with an Ultrabook as it’s a ‘proper’ PC in terms us usability but light to carry.
You could go with the laptops with the removable screen like the Lenovo Helix but these are expensive for what they are (haven’t used one so can’t say how good they are). It may be cheaper to go for a good laptop, a cheap branded Android tablet and an Office 365 subscription.