It might be one of those cases of "too much too soon" since you've only had him since Saturday and all of a sudden he's expected to integrate right into the family, with lots of excitement, your son crying...he's young, he's not settled in yet. I'd dial it back a bit myself, and give him time to adjust. The barking and mounting could have been trying to instigate play - but again, the evaluation will tell you more that my guesses will.
Thank you for giving this pup a home, and a second chance. Maybe the "2 week shutdown" might be in order here?