First off- is it your 5 year old or 8 year old?
I suggest that you read the book "Reading Magic" by Mem Fox. Mem Fox has written a handful of children's books and much more. A lot of the following advice and more is spelled out in her book.
Reading must be a good, positive thing for children. Make reading fun for them. Make it an important thing in your household.
Buy some- at least one or two- books that can be his, that he has read and likes. (Low budget? Go to a thrift store and pick out a few books with him.) Set up rewards for him. Praise him frequently- as he does a good job. Encourage him when he's having a hard time.
Frequent trips to the local library are another budget saver as well. Don't make him read books he doesn't like. If starts a book and he truly doesn't like it, don't make him continue.
Also, read with and to him. Make it a positive bonding time between the two of you.I suggest getting two copies- one you read aloud and one he follows along with. Read some longer books too- especially those with cliffhangers. But instead of reading the whole thing- stop when there's a good cliffhanger. If he really wants to find out what happens, he'll have to keep reading by himself until the next reading session with him.
Put effort and do things with consistency and frequency with him. Be positive, and have fun with him.
Reading is a HUGE foundation to a LOT in life- and as a parent you're responsible for the majority of his learning for life. Children develop mostly after birth, an as the key players in their live you must be proactive!
Good luck and happy reading!