Or he can get more stats but not from thoose items?
I want to know if when he can't used more of thoose items he can still improove in thoose stats.
To clarify, those items boost/add to the EV amount that Pokemon has in that particular stat. For example, Protein, I believe, will give the Pokemon Attack EVs. However, there is a limit to how much you can boost using those items. The rest will have to be manually trained through either Super Training or regular training.
It is not their maximum stat, it is their maximum amount of EVs that can be boosted by that item.
More about EVs here: http://pokemondb.net/ev