I'm at the end of my rope with almost 4 year old dcb.

Bit of background:
Dcb incredibly passive and unmotivated. He is also very defiant in a passive way. He will find any way to get out of what he's been asked to do. I have wondered in the past if he may have a cognitive delay, as he doesn't respond to half of the things I say, but after getting to know him - he's been here a year - and performing two separate screens, I've decided it's behaviour instead of ability.

His parents do close to nothing as far as consistency or following through with demands. If anyone recalls my posts about him (numerous) he will tantrum or just flail about when parents pick him up, refusing to get dressed and mom will just shrug it off and carry him out, no shoes or coat sometimes. When I've tried to step in and help he has an all out meltdown.
He's not toilet trained, though he is well aware of when he is about to go (he's held it enough to pee on his pants rather than the potty, and it always hear him pushing when he poops).
This week I've taken away his meals 5 times. 5 out of 9 meals he's finished early because he hasn't listened to my instructions to sit properly, not bounce on his chair, stop playing with his food (and literally pretending his food is a car on the table, making a huge mess).
Today he woke another child up at nap time, just got up and started touching and talking to her (and they have a light up clock that turns color when nap is over, so he was aware that it was still nap time).

So many small issues of defiance, that add up to a lot of time-outs, and I've had enough.

My issue is, it's completely the parents fault. They have no idea how to parent, and as such, have just been 100% permissive with him. How do I explain this to them? Without being overly rude? No one wants to hear "from what I've noticed at pick-up, You are lacking follow through in your commands, this is likely effecting XXXs behaviour" but that's the truth (and now that I write it, pretty compassionate).

I also would like to set a time frame for improvement, but I'm unsure how to phrase it. Is a month enough time to notice improvements in this area?

Thanks in advance... I'm at my wits end!