Can you draw a number line for him at home?
For the sum you point to the first number on the line, so 5 in the first sum, and then get him to count on 3, then his finger should land on the 8.
Alternatively, another stategy which helps and is good for bigger sums, is to put the biggest number of the sum in his head, so 5 in his head, put up 3 fingers and then count on from the number in his head. So he would point at each finger and then count 6, 7, 8.
A more basic trick is to put dots underneath each part of the sum, so 5 dots under the 5 and 3 dots under the 3, and get him to count the dots together to make 8.
For take-away you can do all of the above but count backwards instead. For the dot method, put 10 dots under the 10 and then cross through 5 of them to leave 5 behind.
You can do this well with pennies/sweets/crayons anything really - get him to draw 5 crayons and then three crayons then add them together to make 8 crayons, for example.
These are very basic sums for y1 ime, so I would say that he definitely has not yet grasped the concept between counting and values yet.
Practice, practice and more practice is the answer I think