Hi, first of all, what kind of tree is it? Thousands of trees are 'bonsai', but without knowing what tree it is, we will have a hard time advising. I do think it's probably one that should be outside, if not all year round, certainly in summer. Yellowing leaves usually means it's getting water more often than necessary, or really, that it's potted in soil that doesn't immediately drain water out but lets it sit there and rot roots... so what kind of mix are you using, something made for houseplants or ?? And (maybe because of the way the picture was taken) it looks possible that the tree is too big for the pot... but don't go redoing anything yet until more info is looked at.

Thanks for the reply, I water the bonsai when the soil looks dried out and usually put it in a bowl full of water. At some point i did repot the bonsai and used organic soil, no special mixture (i have no idea about different potting mixtures). The bonsai is a Ligustrum Liguster, i read they should be strong plant and stand most weather conditions? Do i need to add some kind of fertilizer? I have added another photo to give a better perspective.

Many thanks again for the help!

What you have is a Privet (like the hedge), and it absolutely should be outdoors for life, in a soil mix that has a lot of grit added (like 50%), possibly the tiny gravel you can buy at an aquarium store, along with lots of very small pieces of bark. It does look too large for the pot, but do NOT repot now (bad timing seasonally), and DO find a local bonsai club to show you what to do, how to do it and when. You really need to start reading more about trees, climates, soil mixes for bonsai, pruning, bonsai in general, about a 100 things... you don't 'bonsai' anything and have it live for long without some real knowledge.

Ok thanks for the reply, is there anything i can do in the meantime that would aid it in growing more leaves?

Thanks

Just give it proper care... it's a living thing and won't respond instantly to anything, but could do badly if rushed. It's also no longer spring when growth surges, so let it alone a bit and never water by immersion/submersion, but from above, like nature does.