"LOOK! Tigger!" Jonah bounded like that animal across the floor of Mothercare towards the racks of Tigger playsuits.

They were fleecy, with bright orange and black stripes (of course) and matching hats with furry ears. Jonah was entranced. "We'll get one of those!" he announced. "They'll be too big," I pointed out.

After all, the reason we were in Mothercare was that none of the baby clothes in the flat were small enough for the new baby. At just under six pounds, he was tiny. So Jonah and I had come out to find him something to wear.

I was wrong: they did in fact do a Tigger suit in the 'up to 7lbs' size. But the thought of those strident colours on someone so small was more than I could bear. So I'm afraid I steered Jonah towards the boring plain white cotton outfits and we bought three for baby Theo. We did get a little white jacket too, because Jonah thought it was just right.

In fact, even in those small garments, Theo seemed lost, his limbs curled up somewhere inside the flapping sleeves and empty legs. But for all his diminutive size, he's a normal healthy infant. His parents live just round the corner from Jonah's birthplace, one of the major London teaching hospitals, which has a team of midwives geared up for home delivery. So that was where he was born, at home, in the family flat.

Jonah had been born two weeks late, so it was a bit of a surprise when (as Jonah put it) this baby "popped out of Mummy's tummy" a whole day early. He didn't give much warning of his arrival either - or, fortunately for his mum, take too long about it. Jonah went to bed as usual on Friday evening, and by the time he woke up on Saturday morning, his little brother's cry was there to greet him.

And how does Jonah feel about this new arrival, apart from wanting him to wear orange and black stripes? It's always a tricky moment, when a brother or sister arrives for a child who has been, until then, the centre of his parents' world. It needs handling with great care. So Jonah's mum and dad are making sure he has lots of quality time, moments set aside while the baby sleeps which they can devote entirely to Jonah.

We've done our bit to help him feel he wasn't shut out, too. We bought him a Postman Pat pop-up van as a gift - it's like a sort of playhouse he can get inside, and he clearly loves it. He's had yet another trip to the zoo, and we spent time taking him to the playground and feeding the ducks.

It may be that all this anxious care has paid off. It may be that it wasn't really necessary. There was a moment, on Theo's second day, when Jonah snuggled up beside his mother on the sofa and reached out to stroke the baby's head. "It's all right, your big brother's here," he said, with a mixture of pride and tenderness.

He loves being the big brother. He's eager to help. He wants to find the best toys for Theo to play with - and has his own firm opinion as to which they are. He likes to kiss the baby too, and slip his finger inside Theo's tiny fist. He's obviously fascinated by this strange new creature in his home.

There's another thing, too. He has a new sense of responsibility. Not long ago, he'd had a spell of waking in the night and wanting to get into his parents' bed. But since Theo was born he hasn't even asked. If he wakes at night he settles happily to sleep again, once someone's been in to comfort him.

Somehow, now there's a baby in the house, Jonah seems very grown up. He's someone you can talk to, person to person, have a real conversation with. I suppose we'd all forgotten how little a new baby can actually do. Now we can see how far Jonah has come in his three years. He is indeed Theo's big brother. And in another three years, what then? I can see a day not too far away when Jonah and Theo will be fellow conspirators in all kinds of mischief, the best of friends, just as their dad was - and still is - with his big sister.

* My mother points out that I slipped up in my last Gran column. Her generation of mums may have followed a strict routine in raising their babies, but they were never so heartless as to leave them without feeding for eight hours. It should, of course, have been four hours. Sorry, Mum!