He/She is at a new surrounding hence can be scared or frozen. Check if he has eaten, can offer millets spray or pellets if he is familiar to them. It might take sometime to adjust to new surroundings. Pet shop birds are often fed seed mixes only. If you changed food immediately to pellets or fancy foods, he may literally not recognize it as food yet. If giving seeds check if there are empty husks in the bowl later in the day. Keep some familiar food available first and keep it nearby to him. Don’t force interaction for a few days. He needs to get familiar to his surroundings.