How can I help my child get rid of his severe phobia of spiders?

Therapist. When you talk to your child ask him what makes him afraid of them, what is his worst fear with spiders and is it realistic. Has he had a scary experience? Please do not minimize his feelings, after all there are many of us who don´t specially like the criders. Use all the tools available to you, including a therapist. Be supportive and patient. Young children often have fears.
Exposure therapy. I recommend exploring "exposure therapy." Basically, this is gradually exposing oneself to the stimulus he/she is afraid of. This might start by simply looking at still pictures of spiders and slowly increasing exposure eventually moving to holding realistic toy spiders and ultimately to visiting the local pet store and holding a live spider.