Buy a boneless ham and have it shaved. Buy a rolled rump roast and make Italian beef for sandwiches...re warm just before the party. Buy lots of sandwich buns.
Make a couple of heads of cabbage into slaw, make or buy potato salad. have a vat of veggies and dip...most kids love ranch.
Have a boat load of chips, a tub of ice with sodas and pop and fill your freezer with various cookies.
All very doable for one person....rent or borrow a 10 foot table....set the food out....and there you are! I normally hate the thoughts of paper plates....but I give you permission to use them for this!
And enjoy!
Linda C

Here is a link that might be useful: Italian beef

How about having a 1-4pm party and just ordering (or making) a bunch of long sub sandwiches...slice them small (not everyone will want a full serving)...add some salads, easy desserts, fresh fruit, a few dips, chips, and some cheese and crackers. Your biggest problem may be refrigerator room...at least, that's usually my biggest problem for larger groups. I just have no where to store the cold food stuff!

If the bulk of the 75-100 people are mainly your son's friends, you could consider a pizza party.

I'm doing a graduation party for my DD in June. I'm planning on borrowing a crockpot or two and doing Kalua Pork (see link, it is excellent and very easy!!) and some sort of beef as well (Linda's recipe looks good), slaw, a pasta salad, a bean salad, veggies and dip as well as chips. We should have about 50 people and I do plan on hiring a teen to help out. We are borrowing tables and chairs from a club my DH belongs to. I'm making simple tablecloths from fabric I bought off eBay instead of using the paper ones.

I'll be checking back to this post for more ideas!

Here is a link that might be useful: Kalua Pork