Some chat about this here @julinka
How to use underspend - Budgets - SDS Forum (sdsscotland.org.uk)
I would suggest the starting point is to ask why the full budget is not being spent and to consider whether there are additional unmet needs that you could use the remaining funds on that would improve your adult child’s quality of life. This could be a regular break, attending an activity/event or some technology that would allow him to be more independent, active etc. There are some support planning tools here that might help to explore this: Support Planning | In Control Scotland (in-controlscotland.org)
You might be able to tie these in to the existing outcomes, but if you feel the outcomes are too limited then this might need to be addressed with the social worker. I don’t have experience of managed accounts and their limitations, but hopefully someone else can offer this.