Executor unwilling to obtain probate

Help please: I (solicitor) am appointed as a substitute executor on an estate if the first-named executor (son) is unable or unwilling to prove the will. Mum died November 2021; son is based abroad and is simply not responding to emails, and I think he may be depressed or not wanting to deal with things. The 2-year period for appointing from a discretionary trust is not long away - any thoughts anyone on whether I could get a grant as substitute if the son fails to respond, what I would need to do to justify an application, and when I could make it? I have emailed him and said I would look at doing this if nothing heard by 1 September - no emails received since Dec 2022. I’m not a beneficiary so I don’t think a citation works - ? Thanks in advance for any help.