With LBP you should have no problem. What you need to do is create a new user account/profile on his console, sign in on that one, then link it to your PSN account. Download the content, then go back and unlink the store account and wipe your login/password (or don't save them). Now, I am not sure whether or not you need to keep the system account for yourself, but I am 110% certain you do not need to save the PSN info on the console or have him login with online after you have downloaded the expansions.
At my home, there are three user accounts (mine, my wifes, and my eight year old son). I am the only one with PSN access and I have downloaded everything released for LBP thus far. My son, offline and with his own profile, can use all the costumes, and even play the MGS4 levels and use the paintinator in his custom levels.
