If you help Ad Man by watching a certain number of his videos you'll be able to unlock "Help the Salesman" achievements and get special rewards.
The number above his head represents the number of times you still need to help him before you can unlock one of these achievements!

PS: There are 5 total "Help the Salesman" achievements, separated by the number of times you help him (5 / 10 / 50 / 100 / 500/ 1000 times).