How do I treat this at home or do I need to take him to doctors?

What is the conventional treatment of poison oak?
Treatment consists mostly of protecting the damaged skin, preventing infection and relieving the itching. Over-the-counter medications may be used to treat poison oak. Store-brand or generic versions of these medications also work and are usually available. If the case of poison oak is severe, you should see a physician for more extensive treatment with stronger steroid medications.

Avoid the use of older Caladryl that contains the ingredient "diphenhydramine," unless directed by your physician. Applying diphenhydramine to open sores and taking diphenhydramine by mouth can cause a toxic build-up of the drug.

If you wish, use calamine lotion as a topical treatment.
Aloe vera gel can help heal skin irritated by scratching.
Do not take oral prednisone for poison ivy or oak unless the cause is so severe that you become systematically ill (fever, difficulty in urinating, etc.) Do not use topical steroids unless specifically instructed to do so by a physician.
Change clothing as soon as possible after exposure and handle contaminated clothing carefully to avoid spreading the poison oak. Launder clothes several times before re-wearing.
Never burn poison oak.
Do NOT wash the body with any concentration of liquid bleach after a poison oak exposure. Bleach is not helpful and the vapors can be irritating to the eyes, nose, throat and lungs. Bleach will irritate inflamed skin even more, causing redness and pain to sensitive body parts.

Treatment of Poison Oak - Dr. Weil (http://www.drweil.com/drw/u/ART02773/poison-oak.html)

Tea tree oil, jewel weed work for poison ivy. I don't know if they would for poison oak.