When I did an English Language course we learnt that Ma/Mum and Da/Dad are the easiest things for a baby to say as don't involve much movement from your tongue. So children tend to babble these quite often. If Jamie is associating Ma/Mum with you then i think he's doing well, as thats not just babbling that understanding the meaning.
Imogen said a few words - Mum, Dad, Duck, Hi, Bye for around 10m onwards but generally her speech has been fairly slow. She wasn't that confident and only in the last 5months has really started to talk properly, although its still a bit baby talk