This may be a comprehension gap rather than a motivational issue or memory gap. (See @Anongoodnurse, I can write an answer that doesn't start with physical issues :D )
Math is unique in that it builds concepts in small increments on prior steps. If a prior step was misunderstood, then the student can not advance until the faulty skill has been corrected and mastered. A thorough math skills assessment can identify if he is lacking a prerequisite skill.
Math is also highly subject to handwriting skills. At that age, the most common "math" errors are actually sloppy handwriting, misleading the student into taking a wrong step or obtaining a wrong answer.
What always helped my kids was to get out manipulative blocks and express the skill physically. Legos will do if you don't want to go out and buy a set of math manipulatives.
If you want to test math working memory specifically, you can do an informal test by setting up several lists of random numbers ranging in size from 1 to 10. For each list, read the numbers to him slowly and have him say them back. Start with the smallest set, and progress to the largest. If he gets three sets of the same size in a row correct, advance to the next set. If he fails three sets in a row of the same size end the test.
Most people will max out with a list size of 4 to 5. Gifted people will max out at 7. It is very rare to go beyond 7. I currently max out at between 3 and 4, which is considered low normal.
At 4 or less, it might be worthwhile to visit a psychologist.
A three fraction problem with 5 steps actually has a minimum of 8 distinct memory constructs (three nouns, 5 operations), plus tracking (say 1 to 2 constructs) because he is still learning the skill. If he is gifted and has 7 slots available, and the problem requires 8 slots, then there is simply too much being thrown at him at once. This goes back to the "missing prerequisite skill" I suggested first.
If the problem is motivational, the solution might be to find a real world problem of interest to him that requires that skill. For example, my son refused to memorize addition and subtraction tables. However, once he started playing D & D, he had to be able to roll his dice and add or subtract his modifiers quickly in his head. He didn't realize it was the same skill because it was in a context that was "real" to him.
