how to start introducing milk to baby?

9 months is too early for cow milk, his digestive system isn't mature enough yet to properly digest the proteins. Honestly, some pediatricians, despite being supposed experts on infant and child health, are woefully behind the times when it comes to infant nutrition and don't stay informed on the most up-to-date information. There are pediatricians out there who STILL recommend mixing rice cereal in formula/breastmilk when babies are only 6 weeks old, for crying out loud! It truly is astounding.

Despite what your doctor says, your son does not need cow milk right now, and introducing it before his digestive system is mature enough for it can lead to gassiness and tummy troubles at least, and intolerance to lactose later in life at worst. If you want to give him dairy, plain, unflavored yogurt is a much better alternative (the culturing process removes most of the hard-to-digest proteins). Yogurt and cheese made from goat or sheep milk is even better, since the proteins are smaller. Cow milk in liquid form should wait until past 1 year.

Is it about getting him used to the taste, or to check if he tolerates whle milk well?

The advice his doctor gave you is new to me, but it is true that small amounts of cow's milk are okay at this age as long as it doesn't replace his bottles.

You can use it in the meals you cook for him (for instance a bit of milk in his mashed potatoes), or you can offer a small amount of milk in his cup (or sippy cup, or straw cup, whichever your baby is already used to) with solids instead of the sips of water you've probably been offering.

Since it's not about getting actual nutrients, I would only do this once a day, and offer no more than an ounce or so.

If he's healthy and thriving, I also wouldn't worry too much about doing this regularly. Or at all actually. Like I said, this advice is new to me. I don't know about ANY disadvantages of not offering whole milk yet at 9 months.

With my daughter I started to mix it with her formula. So i'd do half formula and half milk. Although my daughters doctor didn't tell me to have her try milk until at least 10 months old. You could also let her try a little milk like 2 ounces of water and 2 ounces of milk in between her bottles of formula,that works too. Good luck,I hope I helped.

Put a little in a cup and let him drink it. You don't have to do anything special.

9 months seems awfully early, even to be just trying it, but if you insist, don't offer more then an ounce or 2 a day.