and i think that serious is not countable ,, is not that ???
Serious is an adjective, not a noun.
Both are possible.
He is aiming for a more serious approach this time.
More forms the comparitive version of serious. If we say it is more serious, we are suggesting a comparision. As in my example, he is aiming for a more serious approach this time (he is more serious than he was last time).
He has given the offer much serious thought.