Here's what you do.

Take your info off the account (it only shows the first and last four, but still). 

Then change your password.

Then give him the changed password.

Wait til he gives yout he go-ahead that he downloaded and installed it

Change your password back to your normal password.

Get killed by DMJ cause you stole lots of $ from MM.

Make another user on his system then make setup your account on that. Download and install the content and he can play it on his own account. Works for me and my Bro just fine.

Some PSN games I have downloaded is visible to all the users on my PS3, but some arent. Per Sony none should be visible.

Could you just change your password so that your info is still on his PS3 but he can't access it?

Me and a friend do this. He lets me download stuff, then he changes his password so I can't buy anything else, and I can still play the games he gives me (Ratchet & Clank: Quest For Booty for example)

Some games are different though, I believe that for Warhawk you need to be signed in as the account that bought the game.  I'm not sure how LBP handles it.  Maybe try it with something inexpensive like a $0.99 costume to see how it works

just buy a PSN gift card (if they are available in your area). He can use his sign in and you don't have to worry about your account info.

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.

A lot of half information being provided here so here's how to give your little cousin your DLC without giving your account detail

1) Create an account on your cousin's PS3, give it whatever name you want, 'PSN Share' for example.

2) Login to 'PSN Share' and linked the account to your PSN ID, but choose 'Do not save password' as the option.

3) Login to PSN and download the DLC, once it has been downloaded, exit PSN and install DLC.

4) Now you will have 2 account on the PS3, your cousin's account and 'PSN Share' the 'PSN Share' account is linked to your PSN ID but does not have any password so he can't log back in to your PSN account.

Your cousin can play the DLC on any account, but only if 'PSN Share' still exist, if it was ever deleted, you will not be able to access any contented downloaded from it.

Your cousin should not be able to get into your PSN, since there is no password, the local ID is not linked to your own PS3's local ID, its just exist seperately like another terminal.

Hope that helps.