Boxer shorts? T shirt? IMO, that's inappropriately underdressed. The aircraft is a public place. Use the pajamas, and request an extra pair from cabin crew. If there is an extra pair, they'll happily give them to you. And one more thing. I wouldn't have guessed that I have to say this, but one older couple on a MRU-LHR flight in F defied my expectations on this. Regardless of your attire, if you're going to change, do it in the lavatory, not in the cabin.
t-shirt is fine. Boxer shorts are not.
It is highly unlikely that you will get an extra pyjama set. These are strictly controlled in the inventory list of the crew.