My 8-year-old has a problem decoding new words. How can I help him?

Yes, you can help him.   First of all, you need to make sure that he understands all the letters and sounds and what they say when they are in certain situations.  Having a firm understanding of what “d” says every time you see a “d” will make decoding much easier.  If he is used to learning with a sight word based reading system, then sounding out new words will be a new challenge for him as he will be used to memorizing them.  It takes a little of both with our language, decoding skills, and memorization, but having good decoding skills is paramount to successful reading of all words that they come in contact with.  So making sure that he is fully aware of all the letters and all their sounds, long vowel rules, blends, digraphs, etc. will make decoding much easier for him.