"That is certainly a good boy."
Translation:C'est sûrement un bon garçon.
It would only be correct if you added a comma ("c'est un bon garçon, certainement"), but then it changes the meaning of the sentence and it becomes more speculative - like you don't know whether or not he's a good boy, but you hope or guess that he is. Or, it would also be correct in the context of someone just told you a bunch of good things about him but you don't like him much, or you don't know him very well and your first impression of him was neutral or a bit negative, so you're just conceding that point to end the discussion politely.
Basically throwing the "certainement" at the end here gives a negative or unsure spin on the sentence.
(It's been a while since I looked at French, so take this with a grain of salt, and experts please correct me if the intuition is wrong).
Consider that in English,
this/that are used in two different ways.
One way is as a pronoun, to refer to something already mentioned (or implied). For example,
Do you have a hammer? That's the best tool for the job, or
Did you set an alarm? That is a good idea.
that often sounds more natural in this usage, but you can also use
this, for example in
Get 30 minutes of exercise each day. This is the key to a healthy lifestyle. There isn't much difference between
that in this case.
The other way is as an adjective, to describe something further. For example
Give me that hammer, or
Give me this hammer. There is a clear distinction between
that, just like between like
here, in this case.
Perhaps it will help you to understand the French better to think of
ce as the first case, and
ceci as the second.