You are describing the deficit in working memory that invariably is one aspect of ADHD. It will likely help your child if he resumes use of the medication that helped him previously. In addition, you might want to consider the Cogmed Working Memory Training that you may have seen mentioned before on this site. It is a five-week, in-home, computer-based program that utilizes a Personal Coach. Improvement in working memory is assicated with improved attention, better impulse control and improved complex reasoning skills. If you'd like to investigate the program and learn more about it, take a look at Cogmed.com. The site should answer your questions and also point you to qualified practitioners.
I forgot something when I was writing this earlier. Dr. Kennedy has reccomended the Cogmed Working Memory training program to help address some of the cognitive issues you have described with your son. This program apparently can really work and really help children develop working memory, which is one of the skills that is underdeveloped in children with ADHD. If you google for it you will find some good sites with practitioners who can administer this program. I am sure Dr. Kennedy can provide you more information on this program.
You can try to post a reminder board or have his teachers help him write down his homework or hand it in. You can try to sort of do the remembering for him. There is a book called Learning How to Slow Down and Pay Attention-- its a book geared for children with ADHD, and it has some good tips. I read it to my son-- he loves it.
However, it should be noted that the ADHD medication that is given to children is not just to help with hyperactivity, but also with the sort of cognitive diffuculties you describe. You may want to consider putting him back on this medication, or another one. In fact, the cognitive difficulties are the areas of ADHD that are often most helped by medication.