Sean is afflicted with a common baby ailment: GFD-BBS, Generalized Fuss Disorder secondary to Bored Baby Syndrome.
The symptoms of GFD-BBS include crankiness, an unwillingness to be put down on a blanket or the swing or the boppy or the bumbo or anywhere for more than 48 seconds at a time, a focused fascination on one's toes (tasty and delicious!) and of course, dry mouth.
Time, unfortunately, is the only cure for BBS, since once a baby figures out how to work his body and sit up and roll over and have some kind of purposeful control over what happens to him and/or where he goes, he can relieve some of his boredom by, you know, going to do or look at something else through his own free will.
Until such time, though, he needs Mom to take him new places and show him new stuff and it needs to happen constantly, since he's trying to grow some brains and make connections and organize his perceptions and figure stuff out. The world is such a wonderful interesting place, and since he can't travel around to see it, then he must catch a ride on the Mommy-mobile. And he doesn't care if he's in the 97th percentile for weight, it's Mommy's own fault for feeding him, see?
The baby with GFD-BBS is a relentless task master, and will let you know in no uncertain terms when you are failing to perform to his satisfaction. He's also a happy soul--as long as you take him on to his next exciting experience in a timely manner. Otherwise, he might just spit up on you.
Gotta go--time to go find something new and shiny!