The centre-back initially joined the National League side on a four-week deal last month.
However, having made seven appearances for the Cards since then, all parties have now agreed for the 19-year-old to remain at Park View Road until Saturday, January 2.
Thomas has been named in Charlton's matchday squad on five occasions, although he is yet to make his senior debut for the Championship club.
Restore Data
| Team | P | W | D | L | F | A | GD | PTS | |
| C | Portsmouth | 44 | 27 | 13 | 4 | 75 | 39 | 36 | 94 |
| 2 | Derby CountyDerby | 44 | 26 | 8 | 10 | 75 | 37 | 38 | 86 |
| 3 | Bolton WanderersBolton | 44 | 24 | 11 | 9 | 81 | 48 | 33 | 83 |
| 4 | Peterborough UnitedPeterborough | 43 | 24 | 8 | 11 | 84 | 56 | 28 | 80 |
| 5 | Barnsley | 44 | 21 | 12 | 11 | 79 | 60 | 19 | 75 |
| 6 | Oxford UnitedOxford Utd | 44 | 21 | 10 | 13 | 76 | 54 | 22 | 73 |
| 7 | Lincoln CityLincoln | 44 | 19 | 14 | 11 | 63 | 37 | 26 | 71 |
| 8 | Blackpool | 44 | 20 | 10 | 14 | 60 | 43 | 17 | 70 |
| 9 | Stevenage | 44 | 18 | 13 | 13 | 54 | 44 | 10 | 67 |
| 10 | Leyton Orient | 44 | 17 | 11 | 16 | 50 | 53 | -3 | 62 |
| 11 | Northampton TownNorthampton | 44 | 17 | 8 | 19 | 55 | 63 | -8 | 59 |
| 12 | Wycombe WanderersWycombe | 43 | 15 | 13 | 15 | 55 | 53 | 2 | 58 |
| 13 | Exeter CityExeter | 44 | 16 | 10 | 18 | 43 | 58 | -15 | 58 |
| 14 | Bristol Rovers | 44 | 16 | 9 | 19 | 52 | 64 | -12 | 57 |
| 15 | Wigan AthleticWigan | 44 | 18 | 10 | 16 | 59 | 55 | 4 | 56 |
| 16 | Charlton AthleticCharlton | 44 | 11 | 19 | 14 | 63 | 63 | 0 | 52 |
| 17 | Reading | 44 | 15 | 11 | 18 | 63 | 65 | -2 | 50 |
| 18 | Shrewsbury TownShrewsbury | 44 | 13 | 8 | 23 | 33 | 63 | -30 | 47 |
| 19 | Cambridge UnitedCambridge | 43 | 12 | 10 | 21 | 38 | 59 | -21 | 46 |
| 20 | Burton Albion | 44 | 11 | 10 | 23 | 36 | 62 | -26 | 43 |
| 21 | Cheltenham TownCheltenham | 43 | 11 | 8 | 24 | 37 | 61 | -24 | 41 |
| 22 | Port Vale | 44 | 10 | 10 | 24 | 41 | 72 | -31 | 40 |
| 23 | Fleetwood TownFleetwood | 44 | 8 | 13 | 23 | 45 | 72 | -27 | 37 |
| R | Carlisle UnitedCarlisle | 44 | 7 | 9 | 28 | 40 | 76 | -36 | 30 |
| > League One Full Table | |||||||||
