A year is not very long and problems like your son's can take a long time to play out. Quick results, do not necessarily mean instant bounce back and gratitude.
Your son has done really well to be able to kick his addiction so quickly and manage to negotiate reasonable access to his son, but that doesn't mean that the problem is worked through and gone. It may be several years before your son has completely processed the depths to which he sunk and his climb out of it.
And believe it or not, try and believe it, gratitude can be an immense burden for the recipient to process. To have to accept that in adulthood, a married man with a child, he so messed it up that he became a child again dependent on his mother to get himself out of trouble can be an immense burden of shame on his shoulders.
It is nice to think, that he has bounced back after a year of problems and will immediately be full of gratitude for all you did, but he has huge scars on his psyche and they are still red, raw and bleeding. Open appreciation of all you did for him is still someway down the line. Be compassionate and patient with him.