If he's a L4, don't be surprised...the best most L4 seem to get in their tap swing still has their body arched in their back swing with head out. Above horizontal for a L4 either requires an older L4 or a strong L4. Late tap helps of course.
A weak tap swing can sometimes simply be a matter of being weak in general in their shoulder girdle and middle body strength. Weaker kids are often just wiggly in general. Many weak kids tap early because they cannot hold the hollow shape on the back swing with head in as it requires the abdominal strength to keep the hips tucked under.
Some are not also pushing actively on the bar in their backswing. I need to go over this next week and check it. That means shoulders pushed out rather than just hanging.
In all honesty, L4 only requires swings to under horizontal, above 45 degrees. I've got some boys who are trying to swing big on some events but there are too many technical errors that I'd rather take them down a notch.
I'd take a swing 10 degrees above 45 below horizontal that has beautiful shapes rather than a big swing like my boys do now with head out in the back swing and doing the 1/2 turn in arch. Sometimes this is head out in the front swing as well.