Could do with more about what type of jobs he likes to do, and what he needs. By the time most of us reach his age we usually don't actually need more tools as have got all the stuff years ago.
If you are on a limited budget with not much to spend some workshop consumables can make a very welcome present, whatever it is that gets used up on his jobs. Some of the best presents I have got which didn't cost a lot but I enjoy very much are:
Magnifying plastic safety specs, they improve and may save the eyesight i still have.
Disposable gloves for oily stuff, and good for paint and glue too.
Good quality paint brushes, what a treat they are!
Selection of wood screws, self tapping screws and tiny nuts and bolts.
Some rolls of duct tape, and coloured electrical tape.
Selection pack of cable ties.
Big roll of paper towel for cleaning stuff up with.
Cans of WD40, and anti sieze, and spray grease.
Whatever you get your Dad he will surely be pleased with it. The best present you are giving him is showing that you are interested in and value what he is doing, I bet that is what will please him the most.