I am afraid that Legal Aid is not available for the legal side of this - but it is not difficult or expensive to deal with this yourself.
The starting point is to try and discuss matters with the parents using Family Mediation
For which legal aid IS still available
If they agree a Consent order can then be sent to the court confirming that your grandson lives with you which gives you Parental Responsibility so long as he lives with you - until his 16th birthday.
If the mother will not attend mediation then you will need to apply to the court using a Form C100
and paying the fee of £215.
You may be eligible for fee remission
You will need to tick the box asking for permission to apply on the C100 - but you will be successful given all the circumstances
Please ask if you need further details