Hi Bernie,
Don't stress. Birthday parties are pretty similar to when I was a kid. Be glad your DS only wants 3 guests, as this will be a nice number, and you probably know the kids quite well.
Sundew's ideas are great. Here are some more:
Food: Sarnies (cheese, cream cheese, jam, ham all pretty safe bets), pizza, crisps, grapes, carrot sticks, juice cartons
Games: Pass the parcel (one thing that has changed is people generally put a prize in each layer-just something small like a sweet or badge)
Musical statues/bumps
Treasure hunt - this one has been v successful for me - put clues around the house - can be written for an adult to read to them, or pictures (eg draw a picture of the bath and put the next clue in the bath) Emphasise that they are to work as a team. If you do this first the prize could be some chocs in a treasure chest to keep them going until tea.( I'd do at least 10 clues)
Memory Game: the one where you put some itms on a tray, cover with a dish towel, remove an item and they guess what has gone
Timing: Max 2 hour (make it 1 and a half if you're nervous). Most people do 3-5pm, but I quite like 10am-2 or 11-1 as most kids are in a better mood in the morning and your DS won't have to wait the whole day and possibly be over-excited. Don't know if you'll expect other parents to stay, most don't at that age, which should be fine
Party Bag: A lot of people I know are beginning to hate the pesky things (the presents seem to get passed round and round to successive party bags). At my DS2s last party I did a sort of lucky dip with just 1 present each (wrapped up). That worked well . If you do party bags, those Go-Go things are a winner.
Presents: Most people don't open them at the party itself, which I think is a real shame because the birthday boy/girl doesn't get a chance to thank the guest (mind you, that's at parties where the parent has invited the entire class, and that's a whole other thread.....)
Hope your son has a great time, 5 is a lovely age
x