Refusing food, all else well?

Hello, I have a 9 m/o male German Shepherd. For a couple weeks now he has been refusing food, but eating some treats. I have to walk after him and hand feed, so that he eats but even then he's quite reluctant. I tried changing the food, also tried leaving him without feeding for a day to see if there is a change but nothing happened. Everything else seems fine, hasn't had other symptoms. Drinks plenty of water, plays all the time, poo is normal, etc. Recently (as part of another check) we did blood tests and those seemed fine too. Not sure if I have to worry and take him to the vet or is this just the heat/pickiness/behavioural issue?

Hello!  I noticed the phrase 'drinks plenty of water.'  This may be something or nothing, but drinking too much can be even more of a sign of a problem than drinking too little, so it is worth being aware of this. If a dog has been refusing food, out of character, consistently for nearly two weeks, I would be concerned.  How recent were those recent blood tests?  It sounds as though he needs to see the vet, possibly as a matter of urgency (give your vets a call for triage).  Your vet may be grateful for a urine sample caught in a cleaned, dried container on the way.  If blood tests are needed again, you are in a good position as you have recent 'normals' to compare them to.

( Triage is the act of reviewing a new case to see how urgent they feel it is )

Potential problems that may present like this are very diverse.  Your vet may feel that possibilities include liver or kidney disease, infection, pancreatitis, dental disease, diabetes (especially if drinking excessively - I note from your question that this may not be the case); pain e.g. skeletal etc.  Best of luck and please do let us know how you get on.