The parents insistence that his weight isn't a problem,
has nothing to do with his diet and lack of exercise and is purely genetic proves that they absolutely are to blame for his being so fat that he's becoming disabled and that they're too stupid to accept outside help to do anything about it.
Parents aren't entitled to overfed their children into a short life of ill health and limited mobility. I've no doubt that their behaviour is down to lack of intelligence rather than cruelty, they probably don't have the ability to understand what they're doing. The police are required to investigate how much these parents understand and whether they should be held accountable for their abusive actions.
No child be forced to suffer the consequences of incompetent adults, social services don't just exist to protect them from 'evil' carers, they're there to protect children from anyone who endangers or abuses them, including the incredibly dim 'loving' ones. They can't help parents who refuse help or are incapable of understanding what they need to do and they can't abandon children who need help.
I feel very sorry for children of parents like this, they're doomed from birth, every child should have the right to be cared for by somebody capable of loving them, protecting them, who knows what good health is and understands it's importance. If people aren't able to provide that, they can't expect to keep custody of their child. Especially when they're actively causing him serious harm, if he was increasingly physically disabled as a result of his parents 'lovingly' using corporal punishment, everybody would support his removal. There's no difference. Harm is harm.