Hi BabyBlue,
You can hang a bell on his door so maybe you can hear when he opens it.
Do you have a daily schedule? I hate structure but I do know that kids thrive when there is a structured schedule. Maybe try one for a few weeks to see how it goes:
8:00 - 8:30 Breakfast - your son either helps set the table or sits on a chair and visit with you while you get breakfast ready. You can explain to him each step of preparation and perhaps there is something he can help with (like pouring the water into the pancake mix). After you eat, include his help in clean up.
8:30 - 9:00 Exercise time - take a walk, dance, stretches, ect.
9:00 - 9:30 Play time with mom. Have a plan - find an educational game to play - something to learn colors, counting, ect.
9:30 - 10:30 Chore time - vacuum, laundry, whatever. I know with a young helper chores take a lot longer, however, if you don't have anywhere to go what's the hurry any way?
10:30 Snack - make this very nutritional - fruit, yogurt, ect.
10:45 - 11:15 Play time with mom - more educational play
11:15 - 12:00 Free time - though try to have something out for him - playdough, coloring book, pots and pans to make music with, ect. He is overwhelmed with his choices so each day give him something different. This is your time to do your own cleaning or start lunch.
12:00 - 12:30 Lunch time - like breakfast, include him whenever you can -
12:30 Clean up from lunch
1:00 Nap time (or quiet time reading books, watching a children's show, ect.
2:00 - 3:00 Free time - just like 11:15 - 12:00
3:00 - 4:00 Errand time - shopping, etc.
4:00 - 5:00 Chore Time
5:00 - 6:00 play time with mom - educational play
6:00 - 7:00 Supper time
7:00 - 7:30 Free time
7:30 prepare for bed
This is just a sample schedule - each person has to make their own schedule to really work. Everyone's day is different. It gives you an idea of what I am talking about.
When I taught babysitting clinics, I would tell the kids that the more time they spend doing something with their charges, the less trouble the charges will get into. However, as a parent, we also want our kids to learn to entertain themselves not only for our sanity but it is a life skill kids need to learn.
Young children need help in entertaining themselves - they need us to give them ideas. So start free time with one suggestion and after awhile, you can give a choice of two activities.
