You and your husband need to have a chat with him on his own, without the other children listening and discuss this with him.
Ask him why is he doing it and explain to him that looking at other people and kissing in that area in is unacceptable.
You also need to explain to him about hygiene and that poking his finger up his anus can also cause damage as well as spread germs to other parts of himself and especially if he does not wash his hands thoroughly he could end up being ill. Stress the importance of hygiene to him and that he should always wash his hands after using the toilet.
Most boys play with their willies and some more than others. Do not make it a big issue about him touching his own private parts (apart from sticking his finger in his anus) otherwise you may cause more psychological harm to him and this will affect him when he gets older.
Don't shout or punish him, but explain the dangers of poking his fingers into his anal canal, not just germs, but he could damage the anal canal.
Each child is unique and will develop and experiment with his/her own body at different ages. It may be just simply be that he enjoys the pleasure he gets from touching himself and be fascinated by the fact that it firms up. Which it does, even with much young boys.
I would suggest that you encourage him to do more physical sports and encourage him to do extra studies, and take up other hobbies, that way he will have less time to "play" with himself.
Personally, I would NOT have your child seen by a mental health professional. Just keep an eye on him and guide him in the right direction at the time you spot any unacceptable behaviour.
Make sure that you and your husband do give him cuddles and praise him when he earns praise and also give him some quality time without the others. At the age of 9, he is a child and still needs to be shown that you love and care fore him.
Best wishes.
This doesn't sound normal to me especially for a nine year old. My recommendation would be to have him seen by a mental health professional to rule out issues of sexual abuse. It is best to address this issue now by taking him for counseling before this situation becomes a legal issue.