When my friend Catarina turned 40, I made up a box of 40 little things in numbered envelopes, ranging from little cross-stitch to makeup samples, to cards, to fake letters from people. She turned 40 in 2004, so I "sent" letters from both presidential candidates asking her for $1,000,000. She hates McD's, so I sent her a letter for a free McMignon served with McBroth for just $85, and she could supersize it if she wished. I also sent her a letter from Capital One. She had a few weeks before poked her debit card in the receipt slot of the gas pump, rather than the card swipe, and they had to take the pump apart to get it; the letter from Capital One sent her a "replacement" card and it read, "Our motto is 'What's in your wallet?', not 'What's in your receipt slot?'" I also gave her 40 Reasons Why it's Sporty to be 40, listing 40 people who died before their 40th birthdays. (Yes, I'm weird.) I ended it with a paper called "40 Reasons Why Your My Amiga."
You could do fake tickets to next year's Super Bowl (on the "40 yard line"), a can of Super Soup (Campbell's Chunky, of course), little footballs such as are given as child's party favors, a team logo, fake letters from football players, fake invitations to football fantasy camp, etc. And of course, a "40 Reasons Why I Love You" letter. It isn't too difficult to throw such things together. It only took me a few hours to do Catarina's, and the best thing was watching her open those envelopes.
If you don't have time to do 40 items in that manner, I'd suggest Nerf footballs (write "40" on them with a Sharpie marker), little footballs stuck on plastic swizzle sticks (depending on the drink of choice), 40 football logos printed out and stuck taped all over places he might be in the house (bathroom, refrigerator, etc.). You might can get football-shaped candies or candles to put on the cake. As for something to write on it, how about "You just made a first down on the 40-yard line." (I don't know a dang thing about football, so I don't even know if there is a 40-hard line.)
I hope this helps! I'll let you know if I come up with any more ideas.