Tom's initial mass was 56 kg, and his current mass is 63 kg. The increase in his mass is therefore:
```
Δm = 63 kg - 56 kg = 7 kg
```
To find the percentage increase, we can use the formula:
```
% increase = (Δm / initial mass) x 100%
```
Plugging in the values we get:
```
% increase = (7 kg / 56 kg) x 100% = 12.5%
```
Therefore, Tom's mass has increased by 12.5%.