How generous do you want to be? How big is his garden? Without knowing the answer to these questions, I would suggest a strimmer / brushcutter – it will get used most weeks from spring through to autumn, if not several times a week. There are some excellent, light weight models in the Lawnflite range & they have a two year warranty.
For a slightly less generous, but equally as useful present, look at the Agri-Fab range of lawn and garden accessories – spreaders, aerators, scarifiers, carts, etc.