I think I have a question for you: Did you loose anyone in your family (or did a classmate in school suffer a loss?)?
This is striking me more than just the typical separation anxiety. My son (age 5) suffers it a bit when I drop him off at school. We get ready for school in the morning and everything is fine, it is only when I give him the kisses and hugs that he begins to get sad and doesn't want me to leave. He tells my husband and me that it is because he misses me in the daytime.
But what you are mentioning - I am wondering if he is fearing a death of a family member. Whether he heard of it in school, via a story or T.V. show, a dream he had (dreams can be very scary and effect children for a while) or heard you or his father talking of the subject.
He wants to make sure you are safe - that is why I think this. (hence when are you coming home and the side trip making you later.)
Have you talked to your son? I'd recommend sitting down in a comfortable setting and just talk about anything and then lead up to it.
My remedy for my son:
He has a picture of my husband and I in his backpack that he took of us. In addition, EVERY school morning, I tell him to find a special small toy to pack in his back pack to keep him feeling 'safe & secure' while he is in school.
I also have Candy Fluff - a candy scented body powder from Lush that he just loves. It has a few sparkles in it, but unless he has lotion on before I use it- they do not stick to him. On the mornings that he seems to be more sad or distressed, I put it on. We use our imagination and I tell him this is his 'magic dust' that will help him all day and put it on his chest and shoulders (before he puts his shirt on).
I have a few other light scented balms I use from Lush that I wear in the day, and as he misses me, I have him put his sleeves up and apply just a small amount to his upper arm so he has just a hint of my scent on him for comfort as well.
The above tricks work great for us. When I use my scent on him, he has the best days in school! He never takes his small toys out of his bag, but just knowing they are there, really helps him out. Since we have been doing this, he has been getting into his class line for Kindergarten and walking off~ and getting ready to leave almost without his kiss & hug goodbye from me! (so I'm going thru withdraw! LOL~)
And I feel at this age, boys are indeed more attached to 'Mommy' than they are with their 'Daddy'; especially as you work p/t. You are with him more often. It sounds like he has more of a bond with you~
ADDED: Question...Did his behavior start with the new p/t job? I deal with relieving stress as my job, and I know the many triggers. Parents can often overlook the fact that even our kids can become affected by changes. You are leaving the home - that is new.
Again - I recommend talking to him. I have always got on the floor and talked with my son. Actually, I use his puppet (a frog he's had forever) and a small doll (it looks like me, so he calls it 'mini mom') and it helps him to let the information out. He loves this type of interaction and he will ask to do this with me all on his own when he wants to talk. It works great!