The vet couldn't find anything physically wrong with Thompson. He gave me some anti-inflammation medicine to give to Thompson the next few days. The working theory is he might have taken a fall and has lost some of his confidence in his walking ability.
When we got home I was kind of hoping the other boyz would leave him alone but Theo thought it was his brotherly duty to keep his brother company. (Diego-san was also very accommodating during dinner time, not doing his usual nudge Thompson aside and 'share' in more food.)