As a Reception teacher, you have absolutely nothing to worry about at this stage. I don't teach LW, but that's an awful lot of sounds for them to have whizzed through in the autumn term, when they're new to acquiring phonics. It's no wonder he has some gaps.
He has done well to retain most of the individual sounds (and some digraphs too - sounds containing two letters). That's great 👍🏼
About half of my class still struggle to blend independently. Most can do it with adult support, when we sound out together, but quite a few still struggle on their own for an assessment. It sounds like that might be the case with your child, if you though he could blend ok when you read with him.
Blending can be a tough skill to master and children click at different times. He'll get there eventually. Help him by sounding out instructions 'put on your h-a-t', 'put the l-i-d on', 'pop it in the b-i-n etc'.
I'm surprised they have sent assessments home as I'm sure they've done nothing but cause panic and confusion as many parents won't know whether that is good or bad.
In my opinion, he sounds like he's doing ok for this time of year, with extra focus needed on blending. If you are concerned though please do speak to the teacher who will hopefully explain it to you. If they're going to send it out, they need to be prepared to explain it.