I didn't take the time to read your other responses, so I may be repeating what someone else has said. If so I apologize.
I always started teaching my kids to drink from a straw while they were still "on the bottle" as the sucking reflex came natural to them at that time and it was so easy to teach them.
I have however, taught a few who were a little older with some advice my mother gave me.
Use a short cup, like his sippy without the lid (you will have to hold the cup and straw to avoid spills) and fill it with the child's favorite drink, or even a treat like Kool-Aid (to entice him to keep trying).
Cut a straw in half. You want the straw very short so the child will get a reward quickly for sucking properly.
Put something like ice cream or jam on the tip of the straw to encourage him to keep the straw in his mouth and suck on it. You will have to keep putting a tiny bit of the ice cream (or what ever) on the tip, and encourage your child to suck on the straw until he finally gets some liquid through it.
This will take a lot of patience, but when he finally gets that liquid for the first time, the look of surprise on the child's face is priceless ! Make a big deal and praise him !!
Once he has "gotten it", he'll never forget how.
That having been said, I had one child who didn't need to be taught at all.
She was still a bottle baby, sitting up well, but not yet walking. We went into a Kentucky Fried Chicken restaurant to grab a quick dinner. They served us our drinks while we waited for the rest of our order. As always, I put my straw in mine immediately and took a sip, then set it down. I held her in a sitting position on the table in front of me as I turned to speak with my husband. When I turned back around, she was leaning over, drinking from my straw and loving every bit of the Dr. Pepper she was quickly consuming !!! We laughed so hard !! Not only had this child not been taught (yet) to drink from a straw, but she had never tasted soda pop ! I guess she had watched me enjoy mine too many times without offering her a taste, and she wanted to check it out for herself !