Given the pattern of contact that you previously had with your son, you could have objected to her moving, and put in a prohibited steps order application to prevent it - she would have had to explain why it was in the child's best interests to move that far away from his father. However, as she has already moved, it may be a bit late for that.
However, you can still apply for a child arrangements order, and based on the fact that you appeared to have shared care previously, you could make an application for the same pattern.
Firstly you are obliged by law to try mediation to see if that will resolve matters, but if mother won't engage, then you need to attend what is known as a MIAM (mediation information assessment meeting) to get your form signed by a mediator so you can apply to court for an order instead. There are some exemptions from attending a MIAM – the form provides more information on whether you meet that criteria or not.
The form you need is called a C100, which you can find here: https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/734888/C100_eng_0818.pdf
The form needs to be a) signed by the mediator, b) filled in by you, (you are asking for a child arrangements order) and then c) copied three times (with a fourth copy which you will keep for your own records). You must then take or send the original plus 3 copies to the family court closest to where the child lives6
You can find the right court here: https://www.gov.uk/find-court-tribunal
The fee for filing is £215, but you may be entitled to fee remission if you are on a low income or benefits, see here: https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/634329/ex160-eng.pdf
Does that assist?