Duller : The general rule is that one syllable adjectives take -er (though there are cases where
more is also used). So
duller. In this case it seems pretty definitive. I fed both into the
Cobuild Concordancer and got no examples at all of
more dull, but lots of
duller. Here are a few :
a)... The female has
duller, brown plumage, and tends to skulk in the
b)... lost their nerve and gave the job to safer
duller architects" is inaccurate.
c)... the original Ford Escort, which had a
much duller image
than it does today.
d)... about rugby have grown correspondingly
duller
e).... on my evening visit I found the Poissonnerie even
duller.
f)... few of us could find anyone
duller than the prissy John Gummer MP -
g)...when you just look at it but it is actually a
duller white.
h)...they are so much more noticeable during the
duller months of the year.
There are a couple of other interesting things about this sentence, and I hope you don't mind if I refer to them.
Dull : Notice that
dull here has a range of meanings from "not a bright colour" (a, g and h) to "boring" (the others). These meanings often overlap, as possibly in (a) or (h).
However, referring to a person it can also mean
unintelligent, and your sentence is therefore ambiguous - are you saying that he's boring or that he's stupid? For this reason,
dull is often avoided when we mean boring, unless the context makes the meaning clear - as in example (f).
than I : This form is possible, but unusual. The use of the subject pronoun after than is generally considered a mark of formal style, and to me sounds archaic. In everyday speech we would either add the operator*, or use the object pronoun. So :
He is duller than I am or
He is duller than me.
In fact, I'd say the construction with the subject pronoun is virtually non-existent. Again, the concordancer brought up no examples at all, and invented ones -like :
She can swim better than he or
We work harder than they . - sound very odd. They would normally be :
- with the object pronoun :
She can swim better than him,
We work harder than them .
-with the operator :
She can swim better than he can or
We work harder than they do .
* the operator is the verb in the phrase which would be used to make questions, negatives etc
Hope that helps.