It’s pretty common for children at that age to enjoy throwing and/or hitting. As you’ve observed, it’s usually not aggression, can be overexcitement or frustration, and doesn’t mean he doesn’t love his brother. It will pass as his language develops further and he has other ways to express himself. In the meantime….
Introduce the idea of “gentle hands” or “soft hands”, and practice, practice, in the context of play, on dolls, pets, whatever you have available. Teach him what this means. If you think he is hitting because he is excited, you can remind him to be gentle. This will take time and patience.
Introduce games that involve throwing, like a toddler basketball hoop, or Bozo buckets game. If he throws anything else, redirect him to the game where throwing is allowed. Demonstrate concepts like fast/slow, hard/soft, high/low, to increase his body awareness and control.
How much language does he have? If you think he sometimes hits to get something he wants or because his brother takes his things or is in his space, try giving him useful words to use in those situations, such as “that’s mine”, “please move”, or “please give”. Try to catch him in this kind of situation before he hits and gently remind him what to say, or even say the words for him, so you beat him to the punch, so to speak. Give him greeting words too, “Hi G., let’s play”, for situations where he is excited.
If he does hit, focus on comforting the hurt child, and remind T, “hitting hurts, please don’t hit.” Ask him if he can help his brother feel better with a kiss or a gentle touch. Don’t force him, but encourage him to help you. If the hitting appeared to be for a reason, give him better ideas of what to do in that situation. If you don’t know the reason, you can hypothesize, “if you want something from your brother, you need to ask”. Help his brother, who still probably doesn’t care a lot, to give a toy when asked, so that T can see that asking works. If G doesn’t want to give something up, let T know how proud you are of him for asking, you understand he might be sad or mad, and help him find something else cool to play with instead.
Read the book “Siblings without Rivalry,” by Adele Faber and Elaine Mazlish.