I think he did alright. A lot of good artists are just one hit wonders. On the other hand, there have been a number of successful "blue-eyed soul" acts before and after Caldwell including the Righteous Brothers, Hall and Oates, Michael Bolton, and Michael McDonald.
Yes, but one difference is all the artists you site had a base audience that was white/on the pop charts. Hall and Oates did take I Can't Go For That to no. 1 on the soul chart, but all their his were even bigger on the Hot 100. (I just looked it up: Only Do What You Want, Be What You Are peaked higher on the soul chart at #29, than the Hot 100, #39.
What You Won't Do for Love was no. 10 on the pop and AC chart, but made it to #6 on the soul chart. His base audience was the soul audience, which wasn't the case for the others.