After the death of long-term companion, your
little guy probably does miss his buddy. Turtles that live
with the same individuals for a long time seem to form some
attachment. So, he may miss her for a while (I am hesitant to call it
grieving, but...). However, I would't get him a
buddy since there is no telling how they'll get along.
I had a pair of Reeves turtles together for several years.
I had to separate them when the male's advances became too rough.
The female seemed relieved to be rid of him. The male kept
looking for her for quite a while.
(I used to be very skeptical when people attributed human-like emotions
to turtles, but over the years I have learned that captive turtles do a
lot of things that we don't see in wild populations; and emotions, after
are formed in the oldest parts of the brain. While turtles in the wild
don't seem to form special relationships, in captivity, if with the same
turtles/people for many years, they do.)