bk, it is do-able.
Here are my tips. Hope they are not too obvious:
Try to have an early night the night before. You need to feel on top of things to enjoy the travelling. Likewise, leave your destination before tiredness sets in, so you are not exhausted during the journey back.
Definintely take a rucksack type bag. Pack some carrier bags inside - lightweight and good for messy emergencies. They will double as sick bags if the train journey does not agree with your ds.
Do not worry too much about escalators. I have travelled widely with a buggy on the tubes and have found people are very happy to help. Also, lots of tubes have lifts to the platforms. The jublilee line is especially good. Phone up London Transport to find out what stations have lifts. You might be able to plan your route around this. Of course you can't guarantee they will be working on the day
It is possible to balance a pushchair on a single escalator step for either an upward or downward journey. So you won't need any help. Sadly I only got the hang of this 6 months before my youngest son stopped needing pushchairs. I found it too frightening for ages to hold onto the pushchair and keep it level while the lift descended. It can work, though. In the end I did it.
However, it is much easier to hold onto the pushchair while the lift ascends. As long as you can bear the full weight of your child plus pushchair, that is.
Agree about bringing some toys for the train journey. Think about buying a new toy or toys to be extra sure of travelling with a happy son. This is where cheap plastic toys come into their own - lots of visual impact, fun to open and very lightweight so easy to pack away. The surprise element and the unpacking all help while away the time, and that's before you son starts playing with them.
Also take favourite sweets/ biscuits etc as an emergency standby - travelling is a time IMO to relax rules on healthy eating. Pick things that take some time to eat if possible. Ice creams are of course good mood enhancers, especially if the train is hot. Hopefully they will be on sale. That reminds me - pack a plasitc pelican bib!
I can't add more to the taxi advice, and can't think of any other tips for now.
Honestly, you'll be fine, bk. Once you done one trip like this, is will seem so much easier the next time.