It is hard to classify what is normal or average for that age. I had a nephew who was saying single words at 2 at his birthday. He is 15 today and not delayed at all but in honors classes in school.
My second child was extremely chatty. When my 3rd was 17 months old I was watching video of my 2nd at the same age and I began to worry because she wasn't talking nearly as much as my 2nd was. They come around in their own time.
It is great that you are showing him the slideshows. It is beneficial for teaching him to read and to expand his vocabulary. The best thing you can do is talk to him about everything, read to him and sing to him.
It would be great of your older children would read to him and engage him when you are unable to. I a sure you have a foar share of work at the end of the day since you work full time. When my baby was small I had my older children each be responsible for doing a certain activity with the baby whether showing them a book or a set of flash cards or something like that. It is great that you have so mnay helpers.