How old is your ds? What does he feel about all this? - you mention school so I assume he is old enough to have some awareness and opinion
You don't need a solicitor to go to family court. You can represent yourself and ask the court to have the assistance of a McKenzie's friend with you if you wish. If you do decide to use a McKenzie's friend please find one who has a good track record and who is reliable. Or you can ask a friend to be your McK and they will come to court to support you and take notes etc.
How much contact is MIL asking for if your ds is going to loose all his weekends and after school activities? Remind your MIL that contact is for the beneft of ds and shouldn't impede on every other part of his life. A non resident parent could expect to be given contact of every other weekend and half the holidays. Presumably if your ex was still here he would arrange to take ds to see MIL during some of this time, but it is unlikely he would share it 100% with her (unless they live in the same house, and even then he'd take ds out without MIL sometimes). So how much was she seeing ds before?
Your ds does have a right to contact with his extended family, so MIL does have a case to go to court. Think really hard before you write a letter saying you are refusing all contact as that will be used against you in court. Think about what contact you may be willing to offer and why...
Are you prepared to offer contact at a contact centre to the MIL at this point? Google for ones near to you, they tend to have long waiting lists so you/she would have to expect a delay. If not what about offering to send photos and write an update on his progress every 6 or 12 months for her.
The lawyer's letter from MIL will be heavy handed and is designed to make you panic. Keep it to show the judge just what you are being subjected too. Be prepared for her to argue she has no choice given that you are withhold all contact.
Do you have any evidence that she is trying to undermind you as ds's mother and is trying to get custody? Do you have any evidence that she ignores, neglects and is downright awful to your ds? You say it is your opinion, would anyone else hold that opinion on the evidence you hold? Contact os for the benefot of ds, of you have evidence that she is neglecting him or is causing him emotional harm when she has contact then you need to gather as much evidence as you can of that.
Keep a diary of events and copies of all communication. If you have a conversation which needs noting then write down exactly what happened etc. Keep texts and emails. Record any contact if you are there. You don't have to tell her you are doing this. It is legal to do so as long as one of the parties is aware (ie you)
You might want to post in the legal section for more advice on court, your options and what you need to know and do. Please don't panic. Court takes a long time to get anywhere, especially if you are unwilling to allow contact. They will want to see progress, so offering a contact centre may be the place to start. Staff will supervise the contact and report to the court.