So it doesn't sound like it's a true vocal tic. If you think it's related to allergies, that's easy enough to check out at the pediatricians. (Or even, to some extent, just with OTC antihistamines.) You are not reporting anxiety or rigidity, and he has joint attention. That sounds less ASD to me.
Another possibility WRT the "uh" fillers is that he's reached a point in his cognitive development where he's much more aware of the amount of ambiguity in the world, and the difficulty of answering questions comprehensively and with absolute certainty. My EG/PG sibling went through a stage around age 8 (if I recall correctly) where the immediate answer to every question was "I don't know". So much so that we took to using that as a nickname. This from a person who had previously read the daily newspaper cover-to-cover to unwind after a hard day at kindergarten, and read from "A" to "R" (or thereabouts) in the Encyclopedia Brittanica for amusement. In your son's case, he may be using the filler syllables to slow down his response so he can have additional time to formulate a more precisely accurate response, or because he is trying to reconcile what he knows with the infinite amount there is to know.
Reading social situations is the one indicator you mention that is more associated with ASD, but you also have to consider what kind of interpersonal interactions he has experienced on a regular basis. If much of his time is spent with adults, 1) one must be careful to compare his social perception skills with those of an age peer, and not inadvertently hold adult-ish expectations for him; 2) he may not be receiving the same kind of feedback from adults that he would from age-peers, and consequently may not be having the same kind of social learning experiences. Most adults instinctively scaffold for children, which means there are fewer opportunities to compare strategies for their effectiveness, and thus to learn which ones to apply in which situations, with which persons.
I would observe his social reciprocity with those with whom he feels he is on a similar level, and not focus too much on the "little professor"-type conversation that often makes people think of Asperger's. Also, his self-awareness, especially in the social and emotional domains. One of the core deficits often referenced with regard to ASD is a poor "theory of mind"--being able to see from multiple perspectives, particularly to make reasonable estimates of what is going on (invisibly) inside people's heads. If we take as our hypothesis that his poor reading of social situations is a function of his learning experiences, rather than of an essential deficit in theory of mind, then we would expect that he would exhibit more developmentally-appropriate self-perception/self-awareness/insight than other-awareness (due to lack of experience).
Keeping in mind that he is only seven years old, how is his ability to reflect on his own thoughts, feelings, perceptions, and motivations?
