Glad to hear the the wee one is doing so well.
My son did that from a very early age - he was quite content to lie on his mat and wave everything about. We put him under his baby gym from that age and he would play about until he got tired and would just drop off to sleep - I just used to put a blanket on him and leave him in peace. He would wake up later on and just start playing again. It is a very good sign in terms of them being able to amuse themselves and be content. Craig, now 7, will still quite happily play on his own and make up his own games - very self sufficient! His younger brother has been more "high maintenance" you never get two the same, but it is great to have a first one like that as it makes having more easier and they are also very good at making up games for their younger siblings.
Looked in on the boys last night before I went to sleep and Ciar, now 6, was lying flat on his back with his arms up by his head just like they do when they are little babies - it really took me back - he looked so cute!