Hi there.. when you say underweight, how much are we talking about? Cuz a normal 3months GR wt can range anywhere between 7-10kgs and is considered normal for a male.. but if he's below 7kg, you may consider increasing his carb level.. sweet potatoes, paneer, rice food toppers like Henlo from supertails..as means to increasing Jerry's wt. Hope this helps!

Right now he's just 5kg , is it okay! Or I need to increase his diet??

Yes heis little bit underweight. But. It also depends on physical activities and inheritance form father and mother

Underweight does not matter so much bcoz every dog breeding is different if he is healthy and active donot worry. But I will suggest mix home plus food supplement with your home food.