Pip been awarded, when to tell of changes?

Hi my son was diagnosed with Psoriatic Arthritis last July, we claimed PIP in September and he found out last week he has been awarded higher level for mobility and standard for daily living which I think is very fair and what we hoped for.

He is due to start infliximab infusions soon and I'm wondering what to do about PIP if the infusions work for him? (he hasn't had any improvement with DMARDS or Amgevita so we are keeping everything crossed that it does). I know we need to tell them if his condition improves but due to the fluctuating nature of arthritis I don't want them to stop his award then him have a flare and have to start again from scratch.

Does anyone know how long he needs to be well for before we let DWP know he has improved?

Thank you