Re: missing summer classes
That happens more often than anyone thinks, especially when every course have a different starting dates/scheduling. This summer I had one intensive course (1 week, 7 hours a day), a regular weekly course (8 weeks), and an annoying bi weekly course (16 weeks). All started at different times, prompting me to regularly check their starting dates prior to summer just to be safe and sure. Two friends who signed up with me (but in two different courses) were in the position as you're in right now Atra, if not worse. One decided to drop the course, while the other followed through with the professor, got excused, and was offered a make up exam.
Just be honest about it and explain your reasons. From my experience they're too forgiving during the summer, and judging from the email your professor seems nice enough to check up on you instead of not giving a shit.