LB are available at TE depth chart and DT are available at FB depth chart. There may be other positions I just haven't noticed.
You could move the FB version of him to DT to play both offense and defense, and use half his skill points at DT and then edit his position to FB and use the other half at FB to develop in both. Whilst at DT you can sub him in at DE instead... it's not pretty but it's likely the best way to represent that and still play him. Otherwise keep him at MLB and use him as a TE, but progress blocking TE when you use half the skill points.