Recently we went on holiday and he ended up loosing a lot of weight due to high BG levels.
His BG are not that well controlled (we are still learning) what is the best way for him to put on weight? Should he be avoiding eating extra carbs as this could make his levels more erratic? I'm not sure!
Well the best way for your boyfriend to avoid losing weight, and avoiding damaging himself, is to get his BG under better control.
Then if he wants to put bulk on again he could increase his basal dose (overnight or twice daily dose). This will make him hungrier probably and he can "eat to the insulin". Which is kind of the opposite of what would normally be recommended for ideal health, but it should work to put on weight.
But he must get control of the high BG levels that are causing weight loss. That is seriously unhealthy.