Be careful with rhodiola -- it's a stimulating herb for many people, and can aggravate anxiety. Sometimes it helps, sometimes it hurts, and there are plenty of other adaptogens that don't usually stimulate such as ashwagandha and holy basil and siberian ginseng (eleuthero). This herb is in the adrenal support category I mentioned.
The following natural anxiety treatments that have scientific studies supporting their use:
Magnesium
Inositol
Valerian Root
Kava Herb
Rhodiola Herb
Appropriate Exercise (not too much, not too little)
Hypoglycemia Diet
"Food Allergy" Elimination Diet
Candida Yeast Theory (speculative)
There is no generic for Lexapro as far as I know. The generic you're probably talking about is an older drug from which Lexapro was derived, Celexa, but they aren't the same though they are similar. So two things probably happened -- because he didn't taper off Lexapro, he probably is suffering from withdrawal (even tapering can result in withdrawal from these meds) and possibly new side effects from the new med. On the other hand, if his anxiety disorder is "very mild," he should probably not have treated it with these strong meds anyway. Therapy, particularly cognitive therapy, and relaxation techniques are better tried first since if they work the problem is solved, whereas drugs just cover up the symptoms and come with problems attached since they make the brain work artificially. If you need the meds, they're there, but if you don't, they aren't called for. Problem is, this is often the first thing a psychiatrist or doctor will turn to because it's what they do to make money, whereas most therapy nowadays is done by psychologists and other therapists. As for natural remedies, a book to help you is Natural Highs by Hyla Cass, a psychiatrist at UCLA. It's a good place to start, though the book is old. Because natural medicine works on different parts of the system instead of using just one remedy generally, it might be best to see a professional, such as a naturopath or physician who practices integrative medicine. Generally for anxiety they'll go after relaxants and the adrenals and maybe the nutrients that manufacture the neurotransmitters that drugs alter. Good luck