Do I need to report in person for my first 90-day visa registration, and can I do it for my son?

Some people have reported being able to do the first report online with the new system. Usually the biggest thing that can prevent this is if they traveled somewhere in the country and a hotel filed a TM30 for them, but they never filed another TM30 when they returned home, so the 90 day report address isn't matching their last TM30.

You can attempt an unlimited number of times online up until the due date, so just try it and see if it works. If they reject it they should give a reason and you can then try again.

You have 7 days after the due date to file in person without penalty.

As long as you have your son's passport you should be able to do his 90 day report for him.

Try doing it online. It will either let you, or not. If you go your son doesn't have to go with you. In fact you can hire someone to do it, and you don't even have to go. The agent fee runs about bt1000 each.