Five year old boys are at an age where they still enjoy picture books but they're attention  span and vocubulary are increasing so they will enjoy longer stories and may start to like chapter books too. Obviously the best book for a 5 year old boy will depend on his likes, dislikes and his attention span. The article below should cater to many tastes and includes ideas of chapter books, picture books, books about boys and lots more. 

At 5 years old, some boys are at an age where they can be start to read their first chapter books. Reading one chapter a night before bed is often a good idea and is sometimes much more interesting for the adult who's reading it than reading the same picture book over and over again. 
There are lots of great chapter books to get started. Some good authors include Roald Dahl. 
Another top choice is The Magic Faraway Tree by Enid Blyton. Although there is some dated phrases in this book the magical adventures are still as enchanting to children today as they were many years ago. 
Charlotte's Web is another classic book that 5 year old boys will love to have read to them 

Mr Men are good short stories to read to boys before bedtime. Check out some Mr Men ideas below. 

Horrid Henry is a collection of short stories featuring a mischievous boy and some of the naughty escapades he gets up to. I did worry that it would give my own son ideas of naughty things he may do - but not so far and he loves the stories.  

5 year old boys still enjoy picture books and here are a few ideas below. 

Julian Donaldson has written some fantastic picture books. Her style is rhyming prose and her books are really well illustrated. Perhaps her most well known book is The Grufallo which but there are other great books including those below. 

There are so many books suitable for 5 year old boys that it's impossible to mention all the book ideas in one article. But here are a few more top ideas. 
Flat Stanley is a classic book about a boy who becomes flattened by a notice board falling on top of him. However, he is still fit and healthy and manages to go on lots of adventures like going away on holiday by being posted through the mail.
Another Roald Dahl book that is good for this age group is The Enormous Crocodile by Roald Dahl. It's a longer story with 32 pages in total. 
Alexander and the terrible, horrible, no good, very bad day is another great picture book which helps 5 year old boys understand that everybody has one of those days and shows how Alexander gets through it. It is very American in parts, which I don't mind but some people do.