I think Jack is a perfectly good name. (I have a close friend with a four year old named Jack. He's been in preschool and hasn't had any trouble at all!) Leland is a bit unusual, but not too many kids like their middle names no matter what it is!
And about the nicknames that kids will give him . . . kids will find something about EVERY name. When I was in grade school my friend, Debra, was called "Bra." (Which sounds ridiculous now, but she was absolutely mortified and humiliated when we were 10!) And in the very rare cases where a name couldn't be turned into a joke, kids come up with a variety of other gems: Brace Face; Metal Mouth, Four Eyes, Fatso, Booger . . . you get the drift.
Jack is a perfectly wonderful name. It's not like you've come up with something off the wall like Dweezil, Moon Unit, Motorcycle, Tractor or anything else that is destined to force him into therapy! It's also not one of the over-used, super common names like Michael, Christopher and Jacob. AND you're not sticking him with an unusual spelling of a common name that will drive him bonkers having to correct people for the rest of his life! (This happened to my husband - and he dislikes it so much he goes by his initials!)
Jack is a good strong name. If you love it, stick with it. (And keep your sister and friends busy trying to come up with names that kids WON'T make fun of! You'll have lots of fun watching them do this! And ask them about nicknames that will happen if he wears glasses, or braces, etc.!)
(I work one day a week when my husband is off, too! It's nice to wear something other than Mommy-clothes and talk to adults for a couple of hours!)