Answer:
Dave is taxed on $109,500 of plumbing income this year.
Explanation:
Dave is the plumber who worked and got paid $109,500 during the year so he should be taxed for it. The fact that Dave made his clients make their checks payable to his son Steve doesn't change the fact that Steve didn't perform any work and the money is part of Dave gross income.
E.g. Dave could have asked a client to write a check payable to a car dealer in order to pay for maintenance services performed on Dave's car, but it would still be Dave's income.