Somma replaced Leeds midfielder Neil Kilkenny in the 74th minute with his side trailing 2-1 and slammed home a fine volley with his first touch 60 seconds later for his 12th goal in 22 appearances this season, 11 of his outings as substitute.
Grayson said: "He's a great lad. He's never once complained, he understood when I left him out last weekend, he understood here.
"He's got a fantastic attitude, he works ever so hard and is a massive team player and I wouldn't say he's best coming off the bench because he scores for us when he starts as well.
"He appreciates what we've done for him in terms of bringing him to this football club, of what he can be and what we've made him.
"But he's a great lad to work with and I'm sure he'll only get better."
Source: PA