It only takes a little imagination to come up with ideas of what to buy for Father's Day. He might have a hobby, or a special interest so think of something associated with those. If there's nothing physical that you can purchase for his interests, think literally, is there a gift experience that would fit like a music subscription or game download?
What's his favourite tipple? Maybe he would appreciate a fine selection of wines or a single malt whisky he would never consider buying for himself.
It's a rare man these days, that doesn't love his gadgets. Keep an eye out for accessories or updates he would appreciate.