Hmmm...my motto is, "If it ain't broke, don't fix it."
If your son is really in remission only on Lialda, then I believe he should keep taking it, unless it is causing side effects.
Because, yes, I have seen first-hand that when somebody comes off a med, then flares, then tries to go back on it, it might not be as effective as it was, before.
Daughter (22) Dx'd Crohn's 3/06. Misdiagnosed for two years, including by top pediatric Crohn's specialist as stress. No symptoms for the past two years. Recent colonoscopy shows stricture gone but still two spots of inflammation in TI. Used LDN to taper off Entocort. Was on Entocort since April 06. Never had D. 4.5 mg LDN, SCD multivit, homemade yogurt, 5000IU vit D3, 500 mg calcium, 27 mg iron, monthly B12 shots, daily oral B12 1000. SCD diet modified to include potatoes and rice.