First - I am NOT lawyer, or legally trained and this is only my opinion so bear that in mind as what is below might be complete rubbish!
Should you maybe arrange to meet with your ex's parents to find out what they propose?
Obviously, it is good that they realise you/your ds count on the money that was being paid every month by your ex, and it is good that they propose continuing to pay something (would it be the same amount?).
When you see them, say that it has all been such a shock and an emotional time and that you are sorry for their loss. Say that only now are you starting to think of the practical impact on your day to day lives.
Ask if there was a will and indicate that you had been told that one existed. If there is a will, ask how, or if, your ds was mentioned. If so, ask for a copy as the information contained will help you plan for your ds' future. Find out who the executor is, and whether or not there is a trustee.
But, don't give too much away. Mainly listen to what they have to say, and what they propose and don't agree to anything. At the end, you could simply say that there is alot to take in and alot to consider and you need to time to think about it all.
After that conversation, you will know more about whether or not a solicitor is necessary.