Lorraine
You can lodge an appeal upto 13 months after the Decision to lower your brothers DLA award, however, if it is more than a month after, you will need to provide Good Cause as to why the appeal is late, which may not be accepted, although this is rare.
See the following FAQ on how to appeal
How to submit an appeal
To ask for a Supersession/re-assessment due to a change of circumstances you will need to show that your brothers condition has deteriorated since the previous Decision. This may be difficult if that Decision was only a few months ago, and there will be an expectation that you will provide evidence to support this.
To request a Supersession you should contact the DWP office that deals with his DLA claim in writing.
Most claimants will not be assessed for PIP between 2014-16, there is currently no information regarding the order that this will done in.
Gordon