You could call local caterers and give them the specs and see if they can give you an estimate that way you have an idea.
Then again, you are cooking for your boss and their son's bday party.
But you also don't want to put yourself in a situation where other events come up for your boss and he wants you to cook for him.
If it were me, ask him to cover the expenses (meat/charcoal, etc.) and maybe give you a day or two off with pay to recuperate.
If he refers you to one of his friends, then start charging accordingly.
There's a lot that goes into it...are you doing apps? Just the main course? Deserts? Breads? Drinks, paper plates, utensils, etc.
Sometimes you have to "suck it up" in the beginning to reap the rewards later on.
Performer 22 / Copper OTS 22 / WSM 18 / Black OTS 18