Yes. I use only garlic and onion infused oils. I also have a couple of powders that are garlic /onion infusions. The powders are very strong flavored where the oils are fairly lightly flavored in my opinion.
Get garlic infused olive oil.
Get something like this: https://www.amazon.ca/FreeFOD-Garlic-Replacer-FODMAP-Seasoning/dp/B089W1TTT7/ref=mp_s_a_1_3?crid=25KAOABX44ZLN&keywords=garlic+fodmap&qid=1655708335&sprefix=harlic+fodmap%2Caps%2C126&sr=8-3
I find the biggest issue with doing these kinds of simple oven meals to be the taste, because we can’t use traditional seasonings like garlic and onion powders. Finding out about the availability of low fodmap onion and garlic replacers powders was a game changer though. You can use these to season any meats or veggies, and the garlic makes a killer garlic bread!
Lactose is the only fodmap in milk. If he is tolerating it well, it should be fine. Most people with IBS can have non fodmap triggers so if it is suspect as far as being a symptom, you can try eliminating it for a week and see how he feels. I would do one item/category at a time (ie do the lactose free milk on a different week than you do the onion removal) so you can isolate which item is triggering him. For onions, dark green parts of scallions, chives, and onion oil are all safe. There is also this weird powdered garlic/onion oil stuff that you can use in lieu of garlic powder/onion powder which is pretty good. https://www.amazon.com/dp/B089W1TTT7?ref=nb_sb_ss_w_as-reorder-t1_ypp_rep_k4_1_11&amp&crid=2NCX0VGXJYOHO&amp&sprefix=low+fodmap+
Interesting idea! We mostly just use this instead: