Aside from that you could explain to him:
Your body will likely need more calories and nutrients to function than his BECAUSE its larger, just for normal things like respiration, cognitive function etc If you were to suddenly eat the amount he does you'd become ill.
The body's natural instinct is to encourage maintenance of the current body - it's functioning in a short term survival level so "if it ain't broke don't fix it"
So when we reduce calorie intake significantly from what it was, the body reacts by providing hunger cues. One of those cues is low blood sugar which can make you feel really rough (nauseous, dizzy, sweaty, headaches) and we all know there's a very quick easy fix - eat or drink sugar!
Numerous studies show that slow, steady weight loss is most successful, no more than 2-4lb per week
And all that is BEFORE you get into the psychological stuff
As soon as something becomes verboten it's all you can think about, the most successful weight loss plans don't ban any food but instead encourage realistic resetting of habits to "everything in moderation" with occasional higher cal foods and drinks built in, not only true of "addictions" eg when on a high bridge and someone says "don't look down" what's the thing you become obsessed with doing? Does he have something he can't quit easily? Smoking? Gym? Phone? X box? Ask him what would be going through his mind constantly if he had to give that up or majorly reduce his time spent on it?
Slow, steady weight loss is just that - slow! 2lb one week 3lb the next...barely noticeable short term so it's very easy to get frustrated and feel like you're putting a lot of effort in for little gain and lose motivation - again has he had to do anything over a long period of time where the goal was a long way off and hard to visualise?
Habits and lifestyle restrictions can also be factors. Shift workers find it very hard to lose weight because their circadian rhythms are all over the place! People who don't get decently timed breaks, especially in roles requiring physical activity often end up with very low blood sugar and resulting symptoms and are often restricted in what foods and facilities they can access at work. So bad habits are easily formed. It takes iirc 90 days of practising a habit every day for it to become a new ingrained habit. It's hard to create a new habit in the context of a constantly fluctuating routine
Especially important for your dp to learn/take on board properly is that many bad habits/diet patterns are emotionally driven and set in early childhood. Either a high cal food is a reward that you've earned ("good girl you ate all your dinner you can have pudding" "well done for passing that test, treat yourself to an ice cream") or something that was denied to you (so having it is an act of defiance) or simply wasn't available (poverty), in more extreme examples abuse victims can over or under eat to ensure they're unattractive and therefore no longer subjected to abuse, or even to punish themselves.
It can be symbolic of love (very true in my cultural background - Catholic - where all mammy's press guests to accept food & drink far beyond what's needed as a way of trying to make people feel welcomed and cared for, true in many other cultures too).
It's also pleasurable and certain foods & drinks actually create chemical reactions in the body that mimic how we feel when we feel loved.
There's also the associations with happy memories - popcorn at the cinema, candy floss at the fair, hot chocolate on bonfire night...also associating certain items with certain much loved people. Hundreds of threads on mn about the foods & drinks mners remember eating & drinking as children and how much they miss them - very few of the answers on these threads are "broccoli"!!