Rather than asking about dropping a particular medication, present your concern in terms of side effects that you believe are excessive and/or dangerous in the long run (like the weight gain and extreme hunger). This does not come across as though you are challenging the psychiatrist's capability with medications... instead, these are additional concerns that need to be addressed.
With the current concerns... I'd say a psychiatrist is likely to be less concerned directly with the weight gain. But, the extreme hunger, and the low-blood-sugar/behavior links... are definitely a concern. For example - if a kid is consistently overeating, but not getting low blood sugar between meals and therefore no behavior impact, it might be necessary to live with the weight gain to get the other benefits. But... this is a bit "over the top" and VERY hard to manage.
While you are at it, consider whether there are behaviors and/or other challenges that are not being addressed... because, if the psychiatrist is going to look at adjusting medications, they might as well (and good ones will prefer to) look at the whole picture.
For example... Sometimes there is a range of medications for one particular issue (adhd focus, for example)... and each of these also have side effects. Side effects can be cumulative... one appetite-increasing medication, combined with another appetite-increasing medication, might be a bigger problem than either medication alone. Side effects can also cancel each other out... one appetite-increasing medication, combined with an appetite-decreasing medication (such as the ritalin family), can be advantages IF the difficult child needs both medications to treat other symptoms/issues.
"Different drugs" to balance out side-effects is worth doing. Other than really main-stream non-mind-altering drugs, I don't like to use "more drugs" to solve a drug side-effect problem. We needed an old standby to deal with stomach problems due to a medication side-effect - ended up being transient problem (treat it, then fine after 6 weeks...).