Secondly if you should be unfortunate enough to die, there is a good chance that your partner may also die – road crashes are probably one of the more common ways for younger people to die, and I imagine you travel with your partner regularly. In that case your child will still be looked after.
Whilst it is discretionary who the pension fund pays out to it would be unusual for them to ignore your explicitly stated wish. They might if, e.g. you have done something after the date which implies a different desire, if you have split up from your partner and not changed the instruction or if some other complicated reason appears. Or I guess if there was any suggestion she had bumped you off!
AFAIK if there is no stated beneficiary, or for some reason the fund doesn’t want to pay to the stated beneficiary, or perhaps the beneficiary has died too, then the money is still payable but goes to your “estate” and is then distributed either in accordance with your will (if you have one) or the default rules (which probably transfers everything to your son?).
Ignoring the inheritance tax issues, the real advantage of nominating someone is that the pension fund will pay out quite quickly to the nominee. Rather than having to wait for the estate to be sorted out (maybe 6 months) – if a household has just lost its main income this may be absolutely critical. I only have experience of this once, but the pension fund were the first people to sort anything out, and went out of their way to be helpful. If that money had been held in trust for my nephew it would have been a real PITA – my sister in law needed that money; so I certainly wouldn’t put is it all in your son’s name.
I guess if your family (parents, siblings etc) didn’t like your partner then you might be more concerned that in the event of your death they might try to convince the fund to pay out differently; but again my only experience has been that everyone is most concerned about the “surviving spouse and child” not their own gain.
Of course since you are obviously committed to your partner and have a child with her – you could always marry her as it makes these issues much neater!