What age/size for move from flat pram to upright buggy?

[TakeLovingChances]

Right, I'm a reasonably well-educated and informed woman, but for some reason pram/travel systems/buggies etc really confuse me!

I'm a first time mum to an 8 week old DS and need advice from others more in the know....

We have a Silver Cross 3D pram www.silvercross.co.uk/our-range/pram-systems/3D/
and up until now it's served him well when we're out and about. But, we are starting to have a bit of a problem.

DS is quite nosey and likes to look around him, and he's starting to get more and more upset when in the pram (when it's lying flat) and he screams and screams his head off.

I know that this pram converts to a forward facing stroller/buggy thing, but my question is: from what age can DCs use it in this way? Do they have to be a certain age, size and weight for it to be suitable for them? DS is tall for his age and weighs almost 12lbs, but his head control isn't great yet.

I don't think he's enjoying the pram when it's flat and I'd like walks to be a bit more enjoyable for us both and if sitting upright will help him then I'd like to do that.

Advice please?

Zombie thread!

Just a thought for anyone else reading this a while after like me, I bought a reflux wedge thinking reflux might be why she was crying in her pram... It has the bonus that she can see out a bit more whilst still lying sort of flat. The wedge came with a note saying that in hospitals they roll up a blanket and put it in a pillowcase underneath the baby to stop babies sliding down. This has worked a treat. She still cries a bit but I think less. Still not sure whether it's reflux or frustration at a boring view!