Sounds like a long 'business trip'. Technically I think that if you are performing work in a country and contracted for the position you should be taxed in the country you're working in. It would depend if he is a permanent employee of a Belgian/UK company and working here a bit for them or if he was hired as a contractor just to work in CH.

Also you would need to look into the tax treaty between Belgium and CH which would normally cover his type of situation. It should also cover what type of permit he needs, from the sound of it he is on a tourist visa (?) and I would guess that is a big no-no...