How to gain weight?

I guess his food / weight gain, would be related to his activity levels also.

He's having 90g of carbs per day which is less than half the GDA for a non-diabetic. What's the calorie value of his food? - maybe you need to up the protein and fat content of the meals.

Please consider speaking to your doc as maybe there's something else going on to cause the weight loss?

How are his blood sugars? If he was having lots of highs that might cause him to lose some weight perhaps.

When I need to put on weight, I eat more carbs. Maybe he could consider increasing his carbs at each meal and making the necessary adjustments to his insulin (with the help of his team if necessary)? If I get really skinny, I also eat things like spoonfuls of peanut butter between meals and maybe chunks of cheese too. I also add a little olive oil to my food.

The advice above to see a doctor just to make sure there's nothing else going on is good. Always best to check and not assume everything is down to diabetes.

When I was diagnosed I had lost about 15kg. After getting my BG under control I struggled to put on weight and even lost a little more. This was mainly due to not wanting to eat many carbs. Then this year, just under two year since diagnosis I managed to put on weight. I learnt to stop fearing carbs, started eating a little more and accepting I will have to take a little more insulin to cover it. My BG is still good and I have now gained back 5kg.

Hi. 90gm of carbs isn't too low so should be OK as long as he is having enough protein and fat as well. If despite enough protein and fats the weight loss continues then he must see the GP soon.