Looks like a melon of some kind mixed in among tomato plants as those are tomato leaves primarily. So what did you plant in with the tomatoes? Or is it something that just grew out of some compost?

Dave

Stem screams squash. Cant see vine or leaves. If it vines, then a winter type squash.

Looks like a spaghetti squash but if it a volunteer then all bets are off since all types of cross pollinated seeds could have germinated.

What kinds of squash did he eat last year and what might have ended up in his compost, if he has a compost pile?

Looks like Spaghetti Squash to me too. I have a volunteer just like it growing in my garden too. It is the strongest growing/looking plant I have right now with some huge leaves and the only one the deer didn't sample.