If he's adding a single digit number to a larger number, put the bigger number in his head. Use his fingers to 'count on' the single digit number.
e.g. 18 + 6. Put 18 in his head. Count on 6 more fingers, raising or lowering one as each number is counted
19 (finger 1), 20 (finger 2), 21 (finger 3), 22 (finger 4), 23 (finger 5), 24 (finger 6).
This is the 'counting on' method rather than the 'counting all' method (pooling two groups and counting them all) which it sounds as if he has been using up till now
Or, if he is used to prepared number lines, he can draw his own blank one:
Draw a line, put the larger number on it:
__
18
Draw 6 '1 jumps', counting as he jumps or after he is sure that he has drawn 6, so that he gets to the right total in his head. If he needs the security, he can write the number after every jump.
Or, if he knows how many it is to the next multiple of 10 he can simplify that:
18 plus 2 (draw jump of 2) equals 20. That leaves me with 6-2 = 4 left to jump, so that must be 24.
Nothing to stop him drawing his usual counters or rods and cubes either.
If the number to be added is also greater than 10, then the blank number line method will still work OR you can teach him to partition the numbers so that he adds 10 first, then adds the remaining units.
The calculation policies linked to from this page:
schools.norfolk.gov.uk/page_67.cfm?s=1&m=3257&p=1977,index
are quite useful in illustrating what I mean (there used to be an excellent 'progression in addition' paper on the National Strategies site but it has been archived and I can't find it)