A tip would be in order if he looked after another concern, that was not part of the original work order.
If he is there for warranty he is getting paid time and mileage, if he is there on non warranty work he is charging you time and mileage.
I would offer him something to drink and eat if he is there over the lunch hour, as well the use of the facilities.
A tradesman is not generally someone that gets tipped, that said I had a situation last year where I had the tire shop guys go way above and beyond what I expected, I wasn't about to tip each one of the guys $20. I spoke to the owner and asked if they were all adults and as they were I said I'll be back with a few adult beverages for after work. He said that was not required, their policy was to look after the visitors first, I was floored one of the locals came in to pick up his car. Guess what it wasn't ready yet because I came along, his response was oh sorry you got problems we'll have you on your way quick.
