Hi Dorit,
Has he ever slept more than 3hrs in a row? With my daughter I realized that at some point her night feeds were not necessarily hunger but more to nurse to sleep. What I did was wait for her to finish feeding, then not letting her fall asleep at the breast but put her in the crib awake. It took 2 nights were I had to use pu/pd to put her back to sleep am then she slept through the night and also ate considerably more during the day.
For me I wanted to make sure that I feed her when she is hungry but not let her fall asleep during nursing.
During the day, is he getting distracted during nursing? Maybe you can go somewhere where he does not get distracted? Also if he wants to nurse after snacking maybe try to distract him until he is hungrier so he eats more?
Hope this helps :-)