Dealing with challenging behaviour

Our 14 year old just recently received a diagnosis for ASD. We have been dealing with some challenging behaviour for over a year prior to getting the diagnosis and are now feeling exhausted by trying to set reasonable and appropriate boundaries which just keep getting ignored.

Desperate to keep the relationship positive, but also to keep them safe.

Realise we will need to really consider how the diagnosis will impact on how we manage their behaviour. Anyone out there who can offer desperate parents some words of advice?