As usual, polarbear has put it very well, and very thoughtfully.
I'll add that, from the standpoint of effective remediation, although the recommendation of most of the programs of this type (OG, LB, TBT) is for 4-5 sessions per week, in fact, the research says that many kids will derive nearly the same benefit from as few as two sessions a week. (This may or may not be your kid, of course.) So if you're torn between maintaining his progress over the summer and giving him more of a summer vacation, you can pretty safely cut back by half without sacrificing a great deal of reading/spelling progress, and pick it up again in the fall. Especially if you are doing both TBT and LB, which are pretty much the same general approach.
