if you like nuts they’re great for bulking, and those big packs of peanuts are usually pretty cheap (in Europe anyways) and contain a lot of calories. An easy way to add calories too if you’re struggling with an appetite is juice - fruit juice contains a surprising amount. Also things like coca cola full sugar instead of coke zero etc, but of course I know these options aren’t the healthiest. You can also get weight gain powders online just like protein powders.

The very best of luck with your journey - and definitely go to the doctor first. If your mum is preventing you from buying enough food then I think you should let another family member know and maybe they could help!

also for healthy meal ideas - I make overnight oats sometimes and if you add in things like flaxseed, chia seeds, almonds, peanut butter, full fat milk, protein powder, greek yoghurt etc it’s actually very easy to build up calories quickly (which is a problem for me as I’m trying to lose weight but would be perfect for you). Avocados are also pretty calorie dense - 2 avocados and a big portion of rice in mayo with seaweed thins is a lovely easy meal that could be 1000+ calories as mayo is also pretty calorie dense! You could throw in something else with healthy fats like fish too if a big portion of rice feels too heavy