Isaac is two-and-a-half today. It still feels right to celebrate his half-birthdays, so we will be having cake and generally fussing over him for the rest of the day. After all, the six months since his last birthday accounts for 20% of his lifetime to date, and he has changed an awful lot during that time.

Of course, the biggest change over the past six months has been the birth of his brother, Toby. Having been a devoted mummy’s boy right up until Toby arrived, both Heather and I were delighted to see him transform almost overnight into a boy who was equally comfortable and happy in his father’s presence as his mother’s. (Up until that point, Zac made it perfectly clear I was little more than a largely inadequate mummy substitute.) Sure, there are still things where he prefers his mummy – just as there are now activities (such as bedtime stories) which he associates with me – but for the most part he is equally happy with either of us, and does not insist on monopolising our attention in the way he used to.

In fact, he has been brilliant with Toby – talking and playing with him, including him in everything and generally being a caring, sharing big brother. Toby loves it as well; it’s clear to everyone that he adores his brother.

I’ve written elsewhere about his fast-developing language skills. We’ve started to take it for granted that we can now have proper conversations with him without resorting to baby talk, and it’s interesting to see the complexity of his syntax building. It feels like he is absorbing and practising something new every day: most noticeably this week, he has started to drop the word ‘definitely’ correctly into his speech. (Mind you, the last thing you want to hear from him at 5.30 in the morning are the words, “Yes, Daddy, Isaac’s definitely done a poo.”)

And so much of what he says – both the content and the often exaggerated inflection in his voice – is genuinely hilarious (well, it is to me, anyway). In fact, I think I’m going to start up a regular ‘conversations with a two-year old’ series of posts to record them for posterity.

What else? He has become a bit of a technology geek, having intuitively grasped the concept of finding his way around websites by clicking on buttons and hyperlinks, and knowing how to navigate his way around my iPhone and iPod unassisted to find his favourite music videos and YouTube clips. (Seriously, it won’t be long until he is setting the parental controls to keep us out.)

His love for music continues unabated, with a now established pattern of intense obsession over one or two songs for several weeks at a time – his current favourites are Kylie’s ‘Can’t Get You Out Of My Head’ and Ke$ha’s ‘Tik Tok’ – before moving on to something else. He watches videos with intense concentration; he sings (somewhat embarrassingly, he keeps wandering around singing about brushing his teeth “with a bottle of Jack”, as per ‘Tik Tok’); he demands we play the same song over and over again when we’re in the car, which can get quite wearing after a while.

Unfortunately, while he’s a decent enough singer, his dancing leaves a lot to be desired. He has clearly inherited the grace, elegance and nimble feet of his father (sorry, kiddo), who dances like a knock-kneed centipede with only left feet. Which is unfortunate, given that one of his favourite TV programmes (and books) currently is Angelina Ballerina.

(Incidentally, did you know that, through some quirk of evolution, centipedes always have an odd number of pairs of legs which means that, despite their name, they can never actually have 100 legs? You do now.)

He is now really into the concept of holidays, having loved our trip to (don’t laugh) Butlins. Even now, several weeks later, he keeps talking about wanting to go back to “the white tent” (the Millennium Dome-like entertainments pavilion) and asking when we’re next going away again.

Zac really is growing up so fast, and there’s so much to love about the boy he is fast becoming. He still hates football and generally turns his nose up at sports (either watching or playing), but I’m working on that one. In the greater scheme of things, that’s little more than a minor disappointment in a sea of wonderment.

Happy half-birthday, Zac. It’s been a great year so far – and that’s definitely worth celebrating, as I am sure he will remind me later.