There is quite a bit to suggest that he had help, including the distance between the bomb sites and the different types of bombs used. Authorities also believe that he is part of a terrorist cell , so even is he was working alone on this job he is likely not completely alone.
No, he did not act alone. This is a widespread problem that starts with certain ideologies that this person followed. Even if he was the only one who built the bombs, he still has a network of people he met while overseas. He was radicalized by many others who still live.
No, he couldn`t. Suspicion remains that Rahami had outside assistance or training in order to assemble the eight bombs used in the attacks. Despite the New York city police department saying they were not actively looking for anyone else, some experts said the chance no-one else was involved was "infinitesimally small".
No, according to the previous instances of bombing attacks across the world, bombing cannot be a one-man action. The case is obvious - one has to have previous preparations concerning the action itself. On the other hand, the bomb itself is usually hand-made and as such requires an expert knowledge.