Desperately need toilet training help!

DS is almost 4.5 and is STILL having accidents daily. He was toilet trained at 3, so nearly 18 months ago.
He seemed to generally pick wees on the toilet up within a few months, but poos would be hit and miss. He was at a nursery and they worked really really hard with him and by last summer he was pretty much trained, bar the odd accident.
He was fine all summer holidays at home, occasional accidents.
He started a new nursery in September and started wetting and pooing himself daily. Eventually around November time we had 6 weeks of no accidents at nursery and thought we had cracked it.
The day before he went back to nursery after Christmas and he wet himself twice.
Since the start of jan he's had accidents there almost every day, multiple times a day.
On the whole at the weekend he is dry at home.
We've started toilet training again from the beginning, with rewards etc, but he's still having accidents every day.
He started poo holding a few weeks ago and ended up on laxitives from the GP.
Currently he is wetting and pooing himself on average 3 times a day at nursery.
He's started having more accidents at home too, will do half a wee in his trousers and realise and stop himself, or he'll do a 3 second wee on the toilet and then 10 minutes later he'll wet his pants.
I just do not get how a child that was dry and clean 7 months ago can have multiple accidents a day now.
He seems to love nursery, enjoys going, talks about it positively.
I just do not get it.
I have 3 pairs of poo covered clothes that I need to wash daily and I cannot get the stench out of his clothes.
I'm losing the plot.
Please help!