It's because you feed him.
A few tips for you as requested

:
You need to find an avian vet in your area, he will need a new bird check up soon anyway. The avian vet may or may not do bloods but they should do baseline tests on their poop and a physical exam.
Make sure you bird proof the house: no exposed electrical cords, no metals he can chew, no air freshners, no smoking, no perfumes, no polishes or nail varnish, no hairsprays, no strong fumes, no non stick or teflon cookware, no open windows
Bird cage to be cleaned everyday and deep cleaned once per week using f10 or bird safe disinfectant. Household cleaners are poisonous
Large cage- even though he is out of it a lot the cage needs to be big. See minimum sizes and BAR SPACING for lovebirds. Incorrect bar spacing can cause fatal accidents.
Do NOT take your bird outside without a harness or if its in the locked cage. Even the most well trained birds can fly away, get eaten, eat something bad or get hurt
If you're sure about how to use the syringe then that's great but you still need to be vigilent and watch for signs of crop problems. Its not something that's as easy as you think, I've seen it go wrong first hand with the most experienced people.
Crop stasis: when food stays still and does not move through the crop. This can be caused from a variety of reasons but the most common is the food being too cold or thick.
Sour crop: literally like it sounds, a complication of crop statis, the food rots in the crop which results in infection and death if not treated. A vet would literally have to scoop his throat out with a tube and then feed him that way.
Crop burn: Food burns the crop, even if it barely seems warm to you it can still cause crop burn. The crop is extremely sensitive.
Malnutrition: can be caused by inexperienced weaning nd making the food too thin
These are just some ofthe comlications but you will need to visit and avian vet anyway who will explain everything more.
If you tell us where you live in the world we could help you locate an avian vet. I'd get him to duble check the beak colouration and screen for viruses. I'm sure its totally fine and just a mutation of his colourings but it will need double checking.