Little is known about Dadbert, as he is unseen in the comic and only seen (albeit with his face covered) in the TV show in The Gift. The first story about Dadbert, printed in 1995, was that Dilbert and Dilmom lost him at the mall during Christmas of 1992. They never went back to look for him because they were "waiting for a sale."
This story, however, would be reconned a year later, as it would be revealed that Dilmom finally found Dadbert. Apparently, he's been at an All-You-Can-Eat Buffet at the mall since 1989. The problem is, he's such a literalist, he wouldn't leave until he's literally had "all he can eat." And since the restaurant is open 24 hours, it's been a standoff so far. Dilbert never went to see him because he was waiting for "Burrito Night."
Dadbert has yet to be seen or referred to in the strip since.