I am a little confused as to when you should file form W-9.
I was under OPT and now H1B since November. Next yeat I will file as a Resident Alien for Tax purposes.
I need to submit a new W-4 indicating Married filling jointly and my son as a dependent but I read in Pub. 519 that I also may need to file form W-9. Is this necessary and why?

Form W-9 is used to request an Individual Tax Identification Number (ITIN), which is needed to be claimed as a dependent or to file jointly with your spouse.

You file Form W-9 when you file your tax return, one each for your spouse and each child. You also send a notarized photocopy of your spouse's and each child's passport. You put your spouse's and child(ren)'s name(s) on the tax return, but leave the SSN portion blank, complete the return, then mail it to the address on the W-9 form instructions.

The IRS in Pennsylvania then reviews the application, approves it, issues the ITIN, inserts it in the appropriate parts of the tax return, processes the return, then returns the ITIN under separate correspondence to you.

Once you receive the ITINs, you can then file your state tax returns, if needed.

I am still confused. I thought that the form to request the ITIN for my wife and son is the W-7 and the W-9 was just for me.

You ARE correct!

The W-7 is the application for an ITIN!

The W-9 is the form used to solicit someone to reveal their ITIN to you.

My error! Insert W-7 wherever I cite W-9 in my answer!

Sorry for the misunderstanding!