Perfective vs imperfective.
"pomógł" = he helped me in some specific situation.
"pomagał" = he helped me 'generally', therefore either it focuses on the action (he was helping me the whole summer), or he helped me in multiple situations.
In that another exercise you mentioned, "pomagał" would also make some sense.
Here you can only have imperfective "pomagał", as you also have "often".
[Edited] Thanks for your explanation - though perfective vs. imperfective will continue to trip me up for some time yet.
The various past tenses, including their gender sensitivity, are (to me) the greatest obstacles I've so far encountered in Polish. When I first hit them, I seriously considered abandoning the course...