What can I give to my 7yr old to relieve the pain of his ears "popping" while plane is taking off & landing?

Sugar-free gum. Chewing gum, preferably sugar-free gum with the sweetener xylitol, can help equalize the pressure in his ears. Eating or drinking in general can help as well. If he is at all congested, you might also consider a decongestant. Finally, there's always the old standby, tylenol (acetaminophen). Safe travels!
Gum.. I would allow the child to chew gum on take off and landing to help with the discomfort.