Karen, I honestly don't know. He just gradually slowed down a bit with age! he's nearly 11 by the way. We moved around the borough when he was younger and he went to two primaries before getting into his present school aged 7 years old. It's a small school, very dedicted teachers with high expectations of bahaviour - a more calming and disciplined environment than his previous schools. I am sure this helped him settle down, though it took until he was at the end of year 4 to see improvement.
He was never tested for behaviour disorders, though his nursery and reception teacher found him very active and were worried about him. He was on the school's SEN register by the end of year 1. He got taken off it when he changed schools at 7 yeas old but still often had to sit by himself in class as he was so easily distracted.
I don't know why ds was like this in the first place - true our home life when he as 3/4/5/was more stressful (I was working very long hours as well) but even when things were less stressful, and I was around more, his behaviour didn't magically improve for a good couple of years.
I think learning to read really helped him concentrate and be still. He has a pretty active mind but never actually liked toys much so wasn't motivated to sit and play quietly when he was younger. As soon as he could read he got into playing gameboy games, reading books, reading instructions he was more able to occupy himself happily.
HTH - good luck!