The content of this answer is essentially that in user radouxju, but I thought that some précisions and rewording would be useful.
This is one instance of a general construction;
- moi aussi, toi aussi, lui/elle/ça/cela aussi, nous aussi, vous aussi, ils/elles/eux/ceux-là aussi
It is used to associate someone or a group to someone else or to an another group in the way of specifying they have the same characteristic, attitude, behaviour, opinions, etc. Besides persons, things can also be the elements put on the level of an association this way. It is necessary in certain constructions in order to preserve a good style, that is, in the present case, in order to remain entirely unambiguous as to what is being intended, whether association to a group already mentioned or compounding of characteristics. Moreover, even when there does not exist a blatant context of ambiguity, it is used to remove any doubt that could remain in the interlocutor's mind: this is so because the adverb the syntax of "aussi" carries inherently some vagueness as to which element it bears upon in the sentence or said otherwise, as to which element it is connected to. However it is not always necessary to use this construction; whether to use it or not depends on the whole construction.
In the instance of the question, it is necessary but let's look at the following examples;
Je comprends mieux maintenant la réaction de son petit ami, Jean; il était lui aussi très distant.
Je comprends mieux maintenant la réaction de son petit ami, Jean; Jean aussi était très distant.
Nevertheless, in this last example, where we can do away with the added pronoun, it is not considered as bad style to emphasise the association by using it and there are three possibilities.
Je comprends mieux maintenant la réaction de Jean, son petit ami; Jean, lui aussi, était très distant. There is no certitude as to whether there should be commas around "lui aussi", but I'd rather use them.
… ; Jean était lui aussi très distant.
… ; Jean était très distant, lui aussi.