
Unless he is very leggy and light boned 40kg for Leo male his height sounds quite light to me.
My girl is about same height vise and currently weights nearly 50kg and is lean but muscular (about 2 hours of exercise a day).
Mine is a girl and only youngster so I would expect a mature male of the same height weight more than she does(unless he is really light build/boned).
If he is definately healthy (teeth/mouth)and eating some food I would try not to go down the line of hand feeding him.
Some Leos can be picky and will twist you around their paw and make you feed them if they realize you will play this game.
My girl had a stomach problem and was fed home made food(chicken+rice) for a while and as a result she refused kibble.
As we were going to transfer her over to raw anyway we didn't bother forcing her with the kibble...
She happily ate her kibble with very good appetite for 9 months but after having something "better", she was not going to touch the nasty kibble again but would wolf down homemade meat/veg/rice mix.
She would give a bowl with kibble in it a look of discust and then take her mouth full of it and walk off spitting it out of her mouth and push it under the rugs! She would make sure that somebody is watching her show and then go to kitchen and sit in front of the fridge and clearly be asking for "proper food" :)
Maybe you could try different make or maybe you should go and try BARF as suggested before?
Also if you can, you should exercise him more as this would also increase his appetite and I don't think he is currently being excersised enough for a young large dog.