Is this your son's first trip? Have you and the Mrs. been to Disney a few times? If you haven't been much or it's the little' guy's first time, then no. You want at least one full day at the Magic Kingdom. There is plenty to do there for a child and even the grown ups. You won't see it all if you hop. You could probably spend two full days there if you wanted. If you've been to Disney a few times then Park Hoppers are good. We go every few months, so we'll skip stuff like the Tiki Room, Tomorrowland Speedway and Stitch. For a 5 year old, most of the rides at the Magic Kingdom are a must do. There are a lot of classic Disney there. You have character greets with all the classic characters. One thing we've done before is gone to Chef Mickey's. It's gotten too expensive for the quality of the food but you meet Mickey, Minnie, Goofy, Chip and Dale and last time we went they had taken out Donald and replaced him with Pluto. Peter Pan has really long lines so it might be a Fast Pass to consider. We get Space Mountain but with a 5 year old you might want to pass on it or you'll have to do a baby swap.
Honestly, if you're going back in December don't stress too much. What you don't do in February you can do in December. The things you don't care to do again, you can scratch off the list. That's how we play it when we go. There are certain must do's and certain things that if we don't get to it, we don't sweat it because we'll be back.