I would remove all distractions from the room. You might want to leave him with one toy to chew on. The best time to do flash cards is early in the morning possibly when your little one first wakes up, during bath time, after playing, during feeding or right afterwards…etc. It all depends on your child.
If your little one is being very active this does not mean you should not show them flash card. You can turn it into a game. Which is something I had to do with my little one. I’ll would put her in her jumper and go through the cards as quickly as I can. (jump, stop, no, hi… etc) Over and over again. After that I would scramble the cards up an go through them a few more times but this time around i would change the tone of my voice. (mainly to keep her interested) Once were finished I would then act out all the cards we went over. I would say the word jump, jump jump jump. Every time I say it i would jump an she would jump with me. When I want to stop I would say stop. When she sees that I am no longer jumping she would stop as well.