Since he was malnourished you don't want him to gain weight too fast since it will usually just go on as fat. Slower weight gain is ideal since it is converting to muscle. If you are concerned the weight is coming on too slowly adding a bit extra food to his bowl won't hurt him. There is no real expected weight gain it really since it varies by the dog. I am assuming when you got him he had a full deworming regime? Sometimes at the shelter they just get 1 basic deworming when its possible he could need a more complete regime.
Also remember shelters and changing homes are so stressful dogs can drop weight or not gain as quickly.
Since he is 9-10 months he could benefit from some puppy food. You could simply mix it in with the adult food so it doesn't go to waste. He's not nearly finished growing so no need to rush.
Extra treats that are higher in calories like peanut butter kongs etc also wouldn't hurt.