Hi
Please don't panic. By the looks of it, the tooth has snapped at the gum line and is still mostly intact. It will grow back on its own comparatively quickly. Snapped teeth are not all that uncommon in guinea pigs.
Switch from weighing once weekly to weighing daily first thing in the morning for best day to day comparison but ideally you won't need to intervene.
Weight and Weight Loss Explained: BMI, Weighing, Poos and Feeding Support Levels
The remaining tooth should still abrade against the upper teeth although you may get uneven edges until the other tooth is growing up. Incisors are very long (about 4 cm with only the tips visible) and curved, which makes them rather prone to breaking. The broken tooth will grow back within a few weeks but you may want to see a vet if the regrowing tooth is wobbly (indicating another break further down below the gum line) or if your boy is losing weight significantly - over 50g or more.
Otherwise you just have to sit by and wait it out. Once the teeth are meeting again as they should they will rebalance as they abrade against each other. There is no need of any burring of the remaining tooth as long as your boy's weight is still stable because the dental system is still functional with just one missing tooth.
Guinea pig skull with the curved incisors.
View attachment 282088
Nesta's lower incisor after the removal operation (rotten root). You can clearly see how and curved it is.