You are right with the couple of weeks in game.
Still, he managed to adapt to Beccas and Lillys behavior, he went from flirting to fucking with only a few pushes from Becca. But even a couple of weeks in game should make him, if not comfortable, but at least capable to use the word "sex" if he needs to.
All this "Uhmm, you know..." and "I...Uhmmm....that...ahhh...stuff... you know" is not really believable anymore, even after just a few weeks.
Concerning Abby: He´s fucking her "sisters" and keeps it a secret from her. If she discovers this secret, her heart will be broken. He KNOWS that, yet makes no effort at all to solve that problem. On the contrary. He keeps building that secret, knowing well, he could loose Abby if she discovered the truth (his thoughts, not mine).
As far, as we know Abby, she will see this as a major betrayal, since he blocked all her efforts to get closer to him.
I can see his problem with him being that close to her, but I cant understand his actions increasing the rift between him and Becca/Lilli on one side and him and Abby on the other side. It´s just him being afraid to solve that problem consoling himself in complete denial. (See the cemetary incident)
I understand that he is socially awkward, uptight, prude and emotionally constipated. (Been there done that myself.)
But his first to go to method of problem solving, his denial, is way over the top. Even for a few weeks time in game.