Catnip reponse is based on two things...being old enough (the age of sexual maturity, which for some breeds means that first kitten heat or urges, which can be as young as 4 months...kitten toys with catnip are for the human...not the kitten, imho!) and having the correct genes to be catnip responders.
Cats who are not catnip responders may well respond to honeysuckle (be careful - stem/woody portion only, certain parts of the various honeysuckle species are poisonous), and to valerian as well
At this age, it's safe to say your fellow is just not a catnip responder! See if you can find a line of honeysuckle toys to try, there are two that I know of, both I believe have distributors in Europe - Kitty Kottage and Kooky Kat.