My son who is 15 years old work at mcdonalds last year. Do I need to add his w-2 with mine or does he need to file separately?

His W-2 must be filed on his own return if it is more than $6,300. If it is less he is not required to file but it is a good idea to get any withholding. Be sure that on his return someone else can claim him as a dependent is selected since he is your dependent on your return.