It's not uncommon for eight-year-olds to have difficulty with the "r" sound. I would recommend playing sound games with him before you try another approach. If the sound games don't work within three to six months, then contact a speech therapist.
Please be aware that I myself am not a speech therapist. However, I've experienced some success with children who have trouble with both the "r" and the "g" sounds.
The difficulty in getting the correct sound may be the placement of the voice. The "r" comes from the back of the mouth rather than the front like a "t" or an "s." Ask your son to growl deep and low, similar to gargling, in his throat like an angry dog. Have him do this only briefly, no more than five times holding the growl for not much more than five seconds. This growling places the sound too far back, so don't overdo it.
Then have him form his lips correctly for the "r" sound by watching you. Then breathe out the "r" sound without a growl using some words that contain the letter R in different places: Rich, large, and near.
Finally work with tongue twisters. He should not growl while doing the tongue twisters because other letters should not be placed in the back of the mouth. Just ask him to open his mouth wide as he says the tongue twisters to make the vocal muscles work. Around the rocks the ragged rascal ran. Row, Rorie, row. Roar, Ricky, roar.
To gain more control over his voice, your son might be interested in Fred Newman's book MouthSounds. Mr. Newman shows the reader how to make a variety of entertaining sounds, which have nothing to do with the letter "r," but everything to do with learning how the voice works.
If your son continues to have difficulty, I would seek the services of a speech therapist. You may be able to contact the local school district for help, or perhaps your pediatrician can suggest a speech therapist.
God bless you,