That is cool. No doubt you revel in the joy your sons feels and you will enjoy riding with him as he grows. I have seen some little kids on road bikes and some are quite good at it at a tender age.
Don't think any of us forget our first bike. I sure remember mine. Freedom and speed.
Since so much has been written about fit in this thread I will share a nuance that may become blatant to you as you play around with adjusting your new bike to dial in your best position.
- Fit is very subtle. A 3mm change in saddle or handlebar height can spell the difference between comfort and pain.
Saddle tilt and setback are important.
Sabby, experiment with shifter position on the handlebars. Best to do this without bar tape.
Handlebar 'tilt' is very important to hand comfort. I endorse a rotated back position as shown below.
Saddle height is important.
Some pics to instill a visual to help you dial your fit...