I am a piano and violin teacher, and I no longer start piano students before Year 2. I have started younger children in the past, and some of them have stuck with it and played successfully. However, the dropout rate is much higher, I would estimate about three in four will drop it in the first two years, as opposed to around one in four aged seven or more. A failed attempt at starting an instrument can lead them to label themselves as unmusical, thereby putting them off learning other instruments later, when the only issue was their age. Of course, there are always exceptions, but since it is hard to judge where these will be, I now have a blanket rule. I willl start violin students from Year 1, since this involves only reading one line of music, and you can get an appropriately sized instrument.
Reception is too young IMO, children are tired after school, and regular practice is essential if children are to make progress. They also need to be old enough to understand the need for regular practice. As you have a piano, I would encourage him to play about on it, not teaching him to play tunes or to look at music, but to find notes to play 'dinosaur music', 'bird music', 'a storm' etc. He will gain a sense of pitch and tempo if you encourage this, and will be far more ready when he starts proper lessons. (Use prompts like 'elephants are very heavy and slow' to encourage him to use the low notes slowly, or 'is a mouse running up and down the piano' to encourage him to play faster notes).
On a final note, when you do start him with lessons, ask if there is an option for twenty minute lessons when he first starts. Twenty minutes is a long time to concentrate in a one-to-one situation, especially for young children. Then move to thirty minutes when he is ready - sometimes a too-long lesson is enough to put children off playing the piano on its' own.