Hi - exTA (male) here :
I have worked with kids that age who can hardly read at all, mainly due to them coming from homes that don't have or don't value books. Your DS is obviously not in that category, and if he is on purple/gold he isn't exactly very far behind the expected level.
Some children can 'plateau' in ability, effort, socially etc, possibly for reasons not connected with school at all. Is he making good progress in other subjects, or have they slowed as well? Could there be other factors unsettling him : a new sibling; death of a relative, friend, or pet; moving house; domestic worries? Children can be very sensitive to emotional disturbances, and that distracts them from effort at school.
Does he do other activities : sport, art, music etc? Does he play with Lego, and not spend TOO much time on computer games etc?
I think all these variables need to be considered, and even then you may not find a real 'cause' for the 'plateau'. Perhaps reading TO him more, particularly non-fiction if there are subjects that interest and you, or another adult, can share with, him might help. Is it possible he feels you put TOO MUCH desire into him doing well, and, unconsciously he is rebelling against that? (That attitude is more a teenage thing, so be prepared!)
However, I don't doubt that when the time and conditions are right, he will resume his progress, and in due course you will wonder why you worried about it.