I think it is a great idea if he wants one, it sounds as though he'd enjoy it and could put stuff in for carting around. I doubt he'll still be using it at though why not? The days of gender stereotyping toys are gone.
My mum bought my son a teaset when he about three and he loved it.
I also bought my friend's little boy a toy upright vacuum cleaner from Argos when he was little because he always used to get my vacuum cleaner out to play with when he came around. He was so delighted.
When boys and girls play together they play with eachother's toys so if he has a little girlfriend she can put a dolly in his pushchair and they can walk it around the garden.
I'm quite delighted you are considering it. Nothing 'unmanly' about wanting a pushchair. I wouldn't buy him a very expensive one though.
When I was small, parents bought me a very elaborate (Marmet I think), dolls pram and I hated it. Once I was out shopping with my mum, she had put some of her shopping in it and I let go of it with no brake on. It flew out into a main road and upturned! I was severely told off for that but I used to walk around in a world of my own and just didn't think, it wasn't deliberate - and I was only about four. However I was old enough to be acutely embarrassed by the pram.
No, dollies and dolls prams were not my thing but I'm still definitely a woman, never had doubts about that.
My dad always told us stories about how he played with his younger sisters' dolls as a young boy but he grew up to be a footballer.