They are allowed to wear water wings or a life vest at both resorts. My son had no problems on the slide at VWL when he was four but, it you are concerned u could wait at the bottom for him. One thing to note is that you have to lie on your back going down the slide. If you don't - you kind of get thrown backwards anyway! My 2 year old daughter got on the slide herself one day, whilst I was getting towels and got a huge shock when she was thrown backwards but, she survived!
I haven't used Boardwalk pool but, I know they allow them, as children who are staying at the Beach Club are allowed to use the Boardwalk slide, if they need to wear water wings. Water wings aren't allowed at the yacht and beach club, incase the child gets stuck in the tunnel, according to one of the lifeguards.
If you are still worried, you could wait and assist him as he hits the water.