I agree that there are too many Jacks. Most MC's named Jack are more heroic/men of action. Based solely on your original, albeit brief, description I'd choose Wally. I agree with Mrs. Jones though, the character is in your head so you will know what suites him best and feels right. I do not, however, agree that Wally sounds round/slow/pudgy/old because I consider Harry to be in a similar vein to Wally and we all know of a pretty popular book series where the young hero is named Harry...
Ultimately it will be the character that makes the name, not vice versa. Go with what feels right.