Adjusting to a new job after being a SAHM is tough! I know, I've been there. Give yourself a break- but try to get your routine back on track as soon as you can. Your kids need this, too.
Rosebud gave you good advice. I agree, the first part of your routine should be bedtime. Get them in bed by 8:00, no exceptions. Have their clothes ready to go that night (let them help you with this) and find ways to have a good breakfast ready for them when they wake up.
Really, really try hard to have a family meal *most* nights. Considering activities and such, I know this one can be hard, but it really made a difference with my family. This was our time to "regroup" and talk about our days. Tell them about your job and talk about what they did at school.
Get a good calendar and make lists- that way things don't slip by you and your kids will learn that they can still count on you to have things like permission slips signed and treats for soccer practice ready.
You get the idea. Give this a week or two and I bet you will see a difference in your son's attitude.
Finally, let your 1st grader get through a few more months of school. Have a meeting toward the end of the school year and really consider where he is academically and emotionally.
Good luck! You can do this! Keep us posted...