I'm in a similar situation to @stairway on this: my husband is Muslim and wants me to be able to fast with him during Ramadan - especially as it's something we want to do as a family if we have children. The last 2 years, I tried fasting the whole day every day and totally flaked out after a couple of weeks because I found it so hard. This year I'm starting with a shorter fasting time (normal breakfast time around 7am until iftar) and still drinking water (and black coffee, which is my big cheat) during the day. I know my husband would like me to be able to do more, but to me it works better to start with something achievable and gradually extend that, rather than insisting that nothing except perfection counts and then beating myself up when I fail.
I find it interesting that, for example, zakat is accepted as a percentage of savings/income for each individual, but fasting is often seen in quite a black and white, all or nothing way - so that if you don't observe the entire day's fast it doesn't count at all. I find it helpful to remind myself (and my husband!) that I am giving what is possible for me to give in terms of fasting - and that doing more would genuinely compromise my mental and physical health. I know when my husband was a child he started with fasting for a few hours or part of the day and built up gradually. @Yazkiz would it work for your son to do this some days, maybe only doing the full fast on maybe Fridays or alternate days, but doing a shorter / modified "practice" fast on other days? And would your husband be comfortable with the idea of practicing or training for fasting, rather than seeing anything other than the full fast as a failure?
Also, if your son is feeling demoralised because his cousins are fasting every day and he is not, it is worth reminding him that his cousins are living in an environment where everyone is fasting, and that makes a big difference. My husband moved to the UK from the middle East as an adult and he has commented a lot on this difference. In his country of origin, the working day is shorter during Ramadan and everyone gets up much later, for example. He also says that the community spirit around fasting and the general feeling of celebration and goodwill during Ramadan is something that makes it easier and is something he really misses. It's likely to be similar for your son in the UK compared to his cousins. He's seeing the challenge as the same, with him coming up short, but there are a lot of differences that he might be ignoring.