imo, i would get a $20 digital scale tht goes from 0-500, in tenth gram increments, and has a self tare feature so you dont have to include the weight of the container (ive found a small brown lunch sack works good).
imo i would weigh at least bi-weekly and keep a journal. an appropriate scale will tell you whether he is growing or not even when there are no visual indications. imo, sometimes a scale can also give you early warning of weigh loss in a seemingly healthy cham, usually before there are visual signs. also handy for gutloading.
should also be noted that not all xanths attain the same size. imo undersize/underweight xanths are most often are a product of poor conditioning/health of the mother. that being said, if its otherwise healthy, theres no reason an undersize xanth cant live a healthy thriving life, but occasionally they are plagued with poor health.
imo, i would avoid high fat foods (like waxworms) in an effort to put weight on. feeding it high fat diet will only make it less healthy
and compound whatever issues there are. imo diversity in diet is the best way to put on some weight, but theres no guaranty that he will get much bigger at that age, some of the runts of the litter often top out at about 6-7 mths imo, jmo