The thing is, ppl who really know about it this aren't meant to give professional advice without knowing your circumstances.
Greengrows is a little out of date, the last budget made it easier to get money out without the annuity thing.
However, the main benefit of paying into a pension scheme as such is that the contributions are tax deductible against your employment. Now, you are not employed, are you? So you won't get any of that. The real question is, therefore, how should you best invest your money to provide for an income in old age and/or your DSs security.
This is not simple because you do need him to be financially separate from you, otherwise his adult benefits will be reduced. On the other hand you don't want him at risk.
Paying into a pension scheme as such, might not be great value for money in your case, because you will suffer the management charges but not get the tax deductions as you are not employed.
I think you would be well advised to find an investment vehicle that fits your likely future needs. For you it might be property to let out, or it might be equities. I think you should spend a few hundred quid on getting some professional advice.