The feeding tube is always a tought decision. I know for me I felt like I failed or getting it was a punishment. My parents were great through CF, but the only mistake I think they made was telling me "if you don't eat enough you will have to get a feeding tube". Or "if you don't gain this much weight on your own, you will have to get a feeding tube". Every CFer reaches a point that gaining weight, no matter how much you try, just doesn't happen and you need a feeding tube. But I turned out okay so I won't give them too much flak for it.
The feeding tube it difficult mentally, but also important to get during the growing years. All that weight and extra nutrition will just help Sean (and any others) to grow and be on par with their peers as far as height and weight. Day to day is not so bad. Just put the formula in the bag at night and connect at bed and then go to sleep! Disconnect when it's done, and either go back to sleep or start the day.
There are creative ways to hide the tube (I was a big fan of 3M waterproof bandages). Yes, it does irritate the skin, but you can literally live your life and do all of your activities.
The only point I will disagree with through the discussion is when the feeding tube is removed. The hole definitely doesn't close up on its own. I'm sure it's dependant on how long you have it in for. I had mine for 8 years. The GI doctor tried to tell me it would close up in a month. I said you have 4 weeks, if it doesn't close up, you are going in there. I reached my breaking point when my entire cup of Starbuck's coffee poured out of the open hole in my stomach as I drank it. I was in the next day for them to sew up my stomach and then the outside as well. Point for me!
You will be fine. It's the right decision.