Show and critique several episodes of pair programming. He picks up the next task card, does a quick design, and starts writing tests and code. In a couple hours, he integrates his code to the main system, and all tests pass on the first try, just as the rest of the team shows up for work that day. Bob may be a great programmer, but as you might expect, this is not an example of pair programming.
Solo programming can cause several problems.
In the example, Bob may not be having as good a day as he thought - he may be passing tests, but not refactoring enough. The design may work but not be as simple as it could be. Crowd Janna picks her task card and asks Jay for help. Everybody takes a turn, and they all agree it's pretty cool. The code may be fine, but it cost 5 times as much as it should have, and other work was delayed meanwhile.