Attention difficulties can reduce reading scores, and reading disabilities and ADHD are often comorbid.
Given his additional troubles with writing (and yes, this could be attention, language based LD or both), I would refer him for testing. If there is an issue, research has shown that 90% of kids that are caught early and are given specialized reading instruction by grade 1 will learn to read at grade level. Those caught in 3rd grade or beyond often never catch up. Early intervention is best.
You might find this article helpful: http://www.ldonline.org/article/Is_It_a_Reading_Disorder_or_Developmental_Lag%3F
If your child is in a public school make a referral in writing to your principal for a full educational evaluation with emphasis on literacy skills. The district MUST evaluate within approx 60 days and hold a meeting to discuss the results and special ed eligibility. Don't take no for an answer.
The school may offer specialized reading instruction to see if they can get him up to speed without labeling him special ed. Take the instruction but insist they do the testing in parallel.
My son is dyslexic and has ADHD. Certainly the ADHD impacts everything he does, but the reading difficulties are distinct and should be addressed in a specific manner.
BTW - guessing a word based on the first letter and context clues is a very common trait of a reader with dyslexia.