Great questions and thanks for responding."Eat more" is the simple answer.
First question - any medical issues or intolerances that you are aware? No one should be prescribing anything until this has been answered. Specific things I am looking at would be some sort of kidney or liver issues, lactose or gluten intolerances...
Second, let's define the goal a bit. He's 61 kg. What does "add some weight" from a measurement perspective mean to him? Another 20 lbs? 40? How concerned is he with having visible abs? This is a question he needs to answer - it's his physique.
I'll fill in more once these are answered.
Nope, no health issues. Athletic kid and long for just being 5’10”.
I would guess there is some vanity involved regarding the abs, but even if he could add 10 lbs or so before football that would be good. Iowa has summer baseball so until baseball is over, it will be a challenge to focus strictly on strength training and putting on weight.