can't send pics to my sons android

Unless you have an iPhone connected to your iPad, you will only be able to use the Messages app to send messages and pictures to other Apple device users. That error messages is telling you that the person on the other end needs iMessage enabled.

If you have an iPhone, make sure that it's using the same Apple ID as your iPad. Then, go to Settings>Messages>Text Forwarding and enable text forwarding on your iPad.

If you do not have an iPhone, you'll need to email him the pictures.

Meg's last sentence is your answer. You need to email him the pictures.

iMessage is a free internet messaging service by Apple and it only works between Apple devices. Since your iPad has no cellular voice capabilities it cannot natively send SMS/MMS texts. If you own an iPhone, follow Meg's advice and use iOS continutity (Use Continuity to connect your iPhone, iPad, iPod touch, and Mac - Apple Support) to pair the iPad with the iPhone and you can then send and receive SMS/MMS texts via the iPhone connection. Both devices need to be connected to the same wifi network for that to work.

Otherwise, your best option is email.