They won’t expect his behaviour to improve within 3 weeks, if anything you might see things get worse as he starts to feel safer and doesn’t need to hide his distress as much.
The bail address is a matter of public record so if he’s recorded as being there the social worker and safeguarded should know, but tell them both and follow that up by email. Try to link his behaviours back to trauma and to his neurodiversity. Is mum saying why he’s not been assessed? Waiting times are years long so it may be reasonable that it’s not been done yet, but she does need parenting strategies that help his welfare and to support him.
You don’t need to say here, but think clearly about your concerns - are they about your DD, her partner or both. What are your concerns, give examples of what has happened and the impact on your grandchild. Try not to assume or be personal about it by which I mean, for example, you may not like her partner but it’s his behaviour and the impact on your grandchild that’s important. The same for your DD, there may be things you don’t like about how she parents, but it’s the impact on your grandchild that’s important matters and whether it reaches the threshold of causing or risking him significant harm. Don’t forget that because of his additional needs he will be more vulnerable than other children his age.
It may be helpful for you to go through the SHANARRI indicators (google will tell you what they stand for) and think about how you can meet them - that’s the framework services use to assess wellbeing and risk of harm. If you can see differences in him while staying with you, explain these to the safeguarder (for example this is his daily routine and he’s doing well with X), but don’t think it all needs to be perfect because that’s unrealistic. Spend the time you have providing him a safe, stable environment and raise both your achievements and your concerns with the safeguarder, their role is to represent your grandsons best interests at the hearing. They will likely observe him in your home environment, it’s not a test, they’ll want to see how he’s doing and what your relationship is like so just do the things you usually do with him.
I hope that helps a bit, you sound very caring of him.