spew you pay for children once they are 4 on trains in most of europe (including the eurostar) (i think it's 6 in germany though). obviously your DS would not have a seat, although if the train isn't full there is no reason why you shouldn't spread out. we've had 4 of us on two seats in the past, before dd1 was over 4 (including a holiday a fortnight before she turned 4, so she was pretty big. it wasn't much fun, but loads better than paying more seats!)
nb this morning i was booking some eurostar tickets, something went wrong with the booking (i accidently pressed return and it messed it up) then when i went to book it again the website told me that i couldn't book my return leg till 8 may (12 weeks away). this was not true. you definitely have 4 months. this was confirmed when i rang and booked them over the phone (used the international number from my mobile so i didn't have to pay for the call...).
also, if you book with eurostar.com they have a loyalty scheme which gives you £20 off once you have enough points (although this takes longer to achieve if you always manage to get hold of the cheapest tickets iyswim).
and if you are travelling around by train in the UK a lot (sometimes one full-price trip to london is enough to make it worth it) you should get a family railcard - you buy a ticket for your child, even if they are actually too young to pay, but they get 66% off their ticket price and adults get 33% off theirs (if you have a 5 year old too, then there is no need to buy a younger child a ticket, but you must always have a fee-paying child with you).
i am hoping that the daily mail will do their offer again this year where you get a free 2 month family railcard and then a code to get a year-long railcard half price. i missed it last year and i was furious, i've been looking out for it since january this year, i think it will come up in the next few weeks, if it's going to happen. i'll post on here if it does - the free one is definitely worth having, the half price one is only worth if you were planning a long journey by train (although since we always have one, we use it for shorter local journeys too, it just wouldn't be worth buying just for that type of journey).
long post, but i am evangelical about trains, ask me anything!